I've been using an Imp with a Drop Rate of 2 (80) for tests, and that did lead me pretty far but only in terms of collecting items not their original chance of obtaining them.
The DataCrystal's RAM Map is pretty interesting, but it doesn't contain a lot of what's already known in the Tower of Babil Docs and barely anything about battle is even referred to.
For what its worth the exact same "random" sequence of bytes pointed out by Yousei long ago in 1900-19FF are the exact same sequence found in 14EE00 (LoRom/HiRom).
And I have found it! (At least the Item Chance. It was exactly as you thought DeathLike2. 5, 25, and 100%.)
$03/ECE5 BD 8E 35 LDA $358E,x[$7E:358E] A:0000 X:0000 Y:0004 P:envMxdiZc - Load A from Group Item Byte.
$03/ECE8 85 B3 STA $B3 [$00:00B3] A:00B8 X:0000 Y:0004 P:eNvMxdizc - Store A in B3.
$03/ECEA BD 8E 35 LDA $358E,x[$7E:358E] A:00B8 X:0000 Y:0004 P:eNvMxdizc - Load A from Group Item Byte (Why? It's already loaded?)
$03/ECED 29 C0 AND #$C0 A:00B8 X:0000 Y:0004 P:eNvMxdizc - Remove bytes that don't correspond with next checks.
$03/ECEF C9 C0 CMP #$C0 A:0080 X:0000 Y:0004 P:eNvMxdizc - Is it C0?
$03/ECF1 F0 17 BEQ $17 [$ED0A] A:0080 X:0000 Y:0004 P:eNvMxdizc - If so branch to 03ED0A. (Skip the Item Chance - Always Obtain Item.)
$03/ECF3 C9 40 CMP #$40 A:0080 X:0000 Y:0004 P:eNvMxdizc - Is it 40?
$03/ECF5 D0 04 BNE $04 [$ECFB] A:0080 X:0000 Y:0004 P:envMxdizC - If not branch to 03ECFB.
------------------------------------------------------------------------------------------------------------------------------------------------
$03/ECF7 A9 05 LDA #$05 A:0040 X:0001 Y:0004 P:envMxdiZC - Load 05 (5% ) into A.
$03/ECF9 80 06 BRA $06 [$ED01] A:0005 X:0001 Y:0004 P:envMxdizC - Branch to 03ED01
-----------------------------------------------------------------------------------------------------------------------------------------------
$03/ECFB C9 80 CMP #$80 A:0080 X:0000 Y:0004 P:envMxdizC - Is it 80?
$03/ECFD D0 5B BNE $5B [$ED5A] A:0080 X:0000 Y:0004 P:envMxdiZC - If not branch to 03ED5A.
$03/ECFF A9 19 LDA #$19 A:0080 X:0000 Y:0004 P:envMxdiZC - Load 19 (25%) into A.
$03/ED01 85 B4 STA $B4 [$00:00B4] A:0019 X:0000 Y:0004 P:envMxdizC - Store A in B4.
$03/ED03 20 8B 85 JSR $858B [$03:858B] A:0019 X:0000 Y:0004 P:envMxdizC - Jump to Subroutine
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
$03/858B 7B TDC A:0019 X:0000 Y:0004 P:envMxdizC - Transfer Direct Page.
$03/858C AA TAX A:0000 X:0000 Y:0004 P:envMxdiZC - Transfer A to X.
$03/858D A9 62 LDA #$62 A:0000 X:0000 Y:0004 P:envMxdiZC - Load 62 (98?) into A.
$03/858F 20 79 83 JSR $8379 [$03:8379] A:0062 X:0000 Y:0004 P:envMxdizC - (We've covered the subroutine before (above) so I will skip to the new and important dealings.
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
...
$03/838B A6 97 LDX $97 [$00:0097] A:0062 X:0000 Y:0004 P:envMXdizC - Load Random Number from 0097 into X.
$03/838D 38 SEC A:0062 X:0035 Y:0004 P:envMXdizC - Set Carry Flag.
$03/838E E5 96 SBC $96 [$00:0096] A:0062 X:0035 Y:0004 P:envMXdizC - Subtract A ?from 96?
$03/8390 C9 FF CMP #$FF A:0062 X:0035 Y:0004 P:envMXdizC - Is it FF?
$03/8392 D0 05 BNE $05 [$8399] A:0062 X:0035 Y:0004 P:envMXdizc - If not, branch to 038399.
----------------------------------------------------------------------------------------------------
----------------------------------------------------------------------------------------------------
$03/8399 1A INC A A:0062 X:0035 Y:0004 P:envMXdizc - +1 to A (Making it 99)
$03/839A 8D 47 39 STA $3947 [$7E:3947] A:0063 X:0035 Y:0004 P:envMXdizc - Store A in 7E3947
$03/839D 9C 48 39 STZ $3948 [$7E:3948] A:0063 X:0035 Y:0004 P:envMXdizc - Store Zero in 7E3948.
$03/83A0 BD 00 19 LDA $1900,x[$7E:1935] A:0063 X:0035 Y:0004 P:envMXdizc - Load A from faux-RNG at 7E1935+x (again a real random number used to generate a false random number)
$03/83A3 AA TAX A:00CA X:0035 Y:0004 P:eNvMXdizc - Transfer A to X.
$03/83A4 8E 45 39 STX $3945 [$7E:3945] A:00CA X:00CA Y:0004 P:eNvMXdizc - Store X in 7E3945.
$03/83A7 C2 10 REP #$10 A:00CA X:00CA Y:0004 P:eNvMXdizc - Reset Processor Status.
$03/83A9 20 07 84 JSR $8407 [$03:8407] A:00CA X:00CA Y:0004 P:eNvMxdizc - Jump to Subroutine (Random Number Generator - In this case likely between 1-99 (the 63 above))
$03/83B8 60 RTS A:0004 X:0000 Y:0004 P:envMxdizc - Return
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
$03/8592 60 RTS A:0004 X:0000 Y:0004 P:envMxdizc
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
$03/ED06 C5 B4 CMP $B4 [$00:00B4] A:0004 X:0000 Y:0004 P:envMxdizc - Is the value less or the same as the one in B4 (5% or 19%)
$03/ED08 B0 50 BCS $50 [$ED5A] A:0004 X:0000 Y:0004 P:eNvMxdizc - If not branch to 03ED5A. (Cancel Item Obtaining).
$03/ED0A 06 B3 ASL $B3 [$00:00B3] A:0004 X:0000 Y:0004 P:eNvMxdizc - x2 the amount in B3.
$03/ED0C 06 B3 ASL $B3 [$00:00B3] A:0004 X:0000 Y:0004 P:envMxdizC - x2 the amount in B3.
$03/ED0E A5 B3 LDA $B3 [$00:00B3] A:0004 X:0000 Y:0004 P:eNvMxdizc - Load A from B3.
$03/ED10 AA TAX A:00E0 X:0000 Y:0004 P:eNvMxdizc - Transfer A to X.
$03/ED11 7B TDC A:00E0 X:00E0 Y:0004 P:envMxdizc - Transfer Direct Page.
$03/ED12 A8 TAY A:0000 X:00E0 Y:0004 P:envMxdiZc - Transfer A to Y.
$03/ED13 84 A9 STY $A9 [$00:00A9] A:0000 X:00E0 Y:0000 P:envMxdiZc - Store Y in A9.
---------------------------------------------(Looping Point)--------------------------------------------------------------------------
$03/ED15 BF 00 9F 0E LDA $0E9F00,x[$0E:9FE0] A:0000 X:00E0 Y:0000 P:envMxdiZc - Load A from Monster's Item Table.
$03/ED19 99 9C 28 STA $289C,y[$7E:289C] A:00CE X:00E0 Y:0000 P:eNvMxdizc - Store A in 7E289C.
$03/ED1C E8 INX A:00CE X:00E0 Y:0000 P:eNvMxdizc - +1 to X
$03/ED1D C8 INY A:00CE X:00E1 Y:0000 P:envMxdizc - +1 to Y.
$03/ED1E E6 A9 INC $A9 [$00:00A9] A:00CE X:00E1 Y:0001 P:envMxdizc - +1 to A9
$03/ED20 A5 A9 LDA $A9 [$00:00A9] A:00CE X:00E1 Y:0001 P:envMxdizc - Load A9 into A.
$03/ED22 C9 04 CMP #$04 A:0001 X:00E1 Y:0001 P:envMxdizc - Is it 04?
$03/ED24 D0 EF BNE $EF [$ED15] A:0001 X:00E1 Y:0001 P:eNvMxdizc - If not repeat the process until all of the items have been laid out.
The rest of the relevant code is above.
And there it is! All issues of Item Dealings that can be obtained from enemies. You can make the chance to obtain an item as high or as low as you like, but C0 seems to always be 100% because it skips the check to obtain the item. These things are quite simple to change and would likely be quite simple to include into the editor in time.