I don't think it's possible in either game. AI General 2 also uses the AI planner, as does the one for Troy and I presume the ones for other games also.

I've tried to find ways to give the units to the enemy AI instead but have been unsuccessful in both games. I also have asked other modders. If anyone is able to do it, I'd love to know how.

How could I go in and make changes to AI planer?

If you find out, please let me know.. I haven't found a way to do so that doesn't involve creating an entirely new AI Planner. The AI Planner is in the C++ layer, not the Lua layer, so modders don't have access to it afaik. There are moddable files that affect AI decisions, though, so it's always possible that I missed something that would be helpful.

I'm not all that familiar with how AI development works either in this game or in any game for that matter, so forgive my ignorance, but how is the Ally/Reinforcement AI set up? Is it theoretically possible to recreate that AI for the AI General or is that also inaccessible?

@bomba1012 I looked into that approach briefly in the past, both in WH2 and WH3, but I couldn't figure out how to do it that way. (That doesn't mean it's impossible, especially since that approach is outside of my skillset. I don't have much experience modding stuff before a battle starts loading. I also have very little experience with twui files, so I could have missed something while examining the pre-battle UI.)