Mandatory characters

Just wanted to report that I figured it out!

Apparently the subroutine called during the scene at Zozo is a bit different from the one used at other times that the party select screen comes up. If I simply switch out that subroutine in favor of the one that is normally used, I get the desired result.

Just switch this:

B2 25 CB 00 Call subroutine $CACB25

For this:

B2 AF CB 00 Call subroutine $CACBAF

The result is a complete party select screen where Celes and Shadow are forced into the party.

Thanks again! It took some time to get there, but y'all pointed me in the right direction!