Just a thought

It looks like there is actually an unused function that works just like the HP healing Tintinabar, but for MP.
At C0/4A6F it checks for the Tintinabar, and if it is equipped it branches to a jump that goes to C0/AEE8, which is the routine to heal HP (I think).

C0/4A6C: B92316 LDA $1623,Y (Load character's relic 1)
C0/4A6F: C9E5 CMP #$E5 (Is it Tintinabar?)
C0/4A71: F007 BEQ $4A7A (Branch if it is)
C0/4A73: B92416 LDA $1624,Y (Load character's relic 2)
C0/4A76: C9E5 CMP #$E5 (Is it Tintinabar?)
C0/4A78: D019 BNE $4A93 (Branch if it's not)
C0/4A7A: 20E8AE JSR $AEE8
etc.

So I'm guessing that maybe changing that JSR to $A3AF (C0/AFA3) so it jumps to what looks like the same routine but for MP healing might just work... Or it might not work the same at all, considering it's MP, I'm really not sure. Might fill it to max or something on one step. I should test this before even posting...

EDIT: Okay sadly there must be more to it than this, it doesn't seem to do anything for MP and still heals HP, changed two other instances of $AEE8 but no dice. *slaps self for attempting to answer an assembly-related question*