RAM $11E4 and adding Gau to a party?

Looking at the "On that day" script, they just remove him from the party. Since his normal WoR recruitment involves simply going to the Veldt to fetch him, that leads me to believe adding him to the party will fix any Veldt roaming.

to be sure, it also un-enlists (as in, removes them from the roster) everybody besides Celes at CA/5310 - CA/532B.

Verily.  I modified that section to also remove (almost) everybody from the shop lists (to add increased suspense to the World of Ruin, and to hide graphics changes to two of the characters before their re-recruitment).  Part of my event cleanup is making sure all those bits get re-enabled at the right time, but, since I'm setting event bits $2FB and $2EB when Gau is added to the party, I'm hoping it'll work as a straight-up reversal to his removal when the world is destroyed.

If all else fails, I'll throw in a caseword check for him right at the top of the "Find the Floating Continent" segment, forcing the player to go fetch him from the Veldt before confronting Gestahl and the Triad (something along the lines of "We're going to need all hands on deck for this, better go fetch Gau from the Veldt").  Then, instead of unrecruiting him I can just tuck him into an unusable party (like Celes gets put in Party 6 during the opera), and pull him back into the primary party when he's supposed to 'rejoin'.  Since there are no character select screens between the departure and the reunion in my scenario adjustment, that should step around the problem.