Possible solution to the sap death bug

@Assassin
I was able to get xKas to spit out a binary file for me, which I could then transfer to the appropriate addresses in my own file using HxD.  So yay, I know how to do that now.

I had to alter your code to jump to the C0 bank and return long, since I lacked the space to fit it in C2.

It... somewhat works.  Seizure correctly triggers a monster's death counter, but it is not triggering the "Monster HP at or below <X>", the FC 06 battle script command.
I had to remove Imzogelmo's code (the block I shared earlier) to get yours to work, but removing Leetsketcher's Item/Magic counter patch (by coping the vanilla Hex from its associated addresses) made your new function do nothing.  I think there might be problems with my Vargas script, which I tweaked to handle bugs introduced by the latter patch (Leetsketcher's).

Here's my nonStandard Vargas script if you care to look at it.  I have yet to test your patch against any other bosses as I was having trouble here.

Spoiler (Click to View)

Code:

[Script generated by: Final Fantasy 3us Multi Editor, Coded by Lord J]
[FF3usME version: 6.8.0]
[File version: 1.00]
[Section: Monster Battle Scripts]

[script #-1]   ; orig idx=259, "Vargas", nb. bytes=108
FC 14 00 00    ; If VAR000 has all the following bit set: 0
FC 15 00 04    ; If VAR000 has all the following bit cleared: 4
FC 09 05 08    ; If target Sabin (all allies if N/A) is not affected by status Condemned
EF             ; Special
F3 12 00       ; Text: " [I tire of this!]<D>"
F9 01 00 04    ; VAR000 set bit: 4
FE             ; End If and reset targeting
FC 14 00 04    ; If VAR000 has all the following bit set: 4
FC 15 00 02    ; If VAR000 has all the following bit cleared: 2
FC 08 05 08    ; If target Sabin (all allies if N/A) is affected by status Condemned
F9 01 00 02    ; VAR000 set bit: 2
F3 43 00       ; Text: " back,<D>  forward, punch, right?]"
F7 08          ; Trigger event: Sabin must use blitz (instructions follows)
F9 01 00 01    ; VAR000 set bit: 1
FE             ; End If and reset targeting
FC 0B 32 00    ; If timer has reached/passed 50 (locks timer)
FB 00 00       ; has battle timer set to 0
F3 0A 00       ; Text: " [Come on.<D> What's the matter?]<D>"
EE             ; Battle
EE             ; Battle
FE             ; End If and reset targeting
EE             ; Battle
FD             ; Wait until the attack sequence is called upon again, then continue (reset targeting)
F0 E6 E6 E6    ; Rand. spell: GaleCut or GaleCut or GaleCut
FD             ; Wait until the attack sequence is called upon again, then continue (reset targeting)
EE             ; Battle
FF             ; End first wave of attack
FC 12 00 00    ; If following monster is/are dead:
F5 03 01 FF    ; Monsters #1, #2, #3, #4, #5, #6 are killed, from side, indiv.
FE             ; End If and reset targeting
FC 02 5E 5F    ; If monster has been attacked by spell: ChiStrike or Suplex, will target attacker
F7 09          ; Trigger event: conclusion of Sabin & Vargas
F5 03 01 FF    ; Monsters #1, #2, #3, #4, #5, #6 are killed, from side, indiv.
FE             ; End If and reset targeting
FC 06 36 55    ; If target self has less or equal than 10880 HP
FC 15 00 00    ; If VAR000 has all the following bit cleared: 0
F9 01 00 00    ; VAR000 set bit: 0
F3 42 00       ; Text: "  Off with ya now!]<D> "
F7 07          ; Trigger event: Sabin intro defy Vargas
FE             ; End If and reset targeting
FF             ; End

Setting seizure and standing around to wait lets Vargas bypass the 'Off with ya now!' event.
I think Vargas is a good monster to test these things on because he's got a lot of moving parts: he's got an in-battle event, a timer, a targetability toggle, and several HP checks (well, Vanilla Vargas has several, I truncated it to one).

EDIT: If struck once by anything (even a spell that does nothing, which my status monster casts IF a status is present) after Seizure is applied, the script will run as expected. So there seems to be an edge case where Seize is inflicted and then no other damaging exchange occurs.