I felt sure I had found the bit that determines whether battles are magnetic, within the location property data. These are 13 bytes each, and the most significant bit of the last byte is only active in the Magnus Cave, the Agart well, and Adamant Grotto (and the very last dummy map). But I changed it, and nothing happened.

So, the question is, what do these locations have in common? It's the only thing I can think of. The Agart well could be a mistake, since its map number comes right before the Magnus Cave. Adamant Grotto would be a neat place to have a magnetic battle, I think, and perhaps that's what they intended. Except, that bit doesn't appear to be for magnetic battles.
Here's the data, in case anyone is interested (for the Magnus Cave entrance through Adamant Grotto). It starts at AA5A1 (FF2us ROM with header):
8C 02 8C EA 02 10 25 95 86 00 47 B9 B8
8D 02 8D 89 02 10 25 95 86 00 09 BB B8
8E 02 8E 89 02 10 25 95 86 00 80 BC B8
8F 02 8F EA 02 10 25 95 86 00 0A BE B8
90 02 90 89 02 10 25 95 86 00 80 BE B8
91 02 91 89 02 10 25 95 86 00 80 C1 B8
92 02 92 89 02 10 25 95 86 00 80 C2 B8
93 02 93 EA 02 10 25 95 86 00 0B C2 8E
4A 07 94 72 07 40 2D AF 86 00 18 C3 80
95 02 95 66 02 00 00 95 86 00 80 C3 04
96 0E 96 01 0E 10 25 23 86 00 15 C3 03
97 06 97 00 06 10 09 23 86 00 3A C3 0C
98 0A 98 2F 0A 10 19 9B 86 00 4B C3 1C
99 0A 99 2F 0A 10 19 9B 86 00 01 C4 1C
9A 0A 9A 2F 0A 10 19 9B 86 00 02 C5 00
9B 06 9B 2F 06 10 00 9B 00 00 80 C5 1C
9C 0A 9C 2F 0A 90 19 9B 86 00 03 C5 1C
9D 0A 9D 2F 0A 10 19 9B 86 00 04 C5 0C
9E 0A 9E 2F 0A 50 19 9B 86 00 05 C9 0C
9F 0A 9F 2F 0A 70 19 9B 86 00 06 C9 34
A0 0E A0 6E 10 10 25 23 86 00 5B C9 B8And the meaning of these bytes:
00: Map Index
01: Tileset Index
02: Map NPC Layout Index
03: Background Graphic
bits 0-6: Background Graphic Index
bit 7: ?
04: Map Palette Index
05: NPC Palette Index
06: Music Index
07: Map Layout Index (scrolling tiles?)
08: Background Movement
bit 0: Scroll independent of character movement
bits 1-3: ?
bits 4-5: Direction
00: Up
01: Right
10: Down
11: Left
bit 6-7: Speed
00: None
01: Slow
10: Medium
11: Fast
09: ? 80: part of ending / other auto-events?
0A: Map Name Index
0B: Treasure Contents Index
0C: ?
bits 0-6: ?
bit 7: Battles have Magnetic Effect?
Edit: I tested all of the location properties, and I'm sure the metallic property is not contained within them. That means it's tied directly to the map index, so there's something somewhere else specifying it. Too bad.