Disable magic while still allowing Esper equip

The place to start with this would be the menu bank, C3.

If you change C3/4D78 from 02 to 00 with a hex editor, it should make anyone with the fight command capable of equipping an Esper (except Gogo who has a special condition). That includes guests, though, but as far as I know there are protections to keep them from actually writing to the magic ram (which would otherwise corrupt configuration settings). You might run into a problem if a guest equips it and then leaves the party, though, but you can add an event to unequip them if that happens (or change C3/4D71: D0 04 {BNE} to B0 04 {BCS} so that all characters Gogo and higher can't equip espers).

EDIT: Branch if carry SET, not clear. Doh, I keep making goofy mistakes. Branch if carry clear would make it so that ONLY Gogo and guests could use espers, which is bad.