Assuming that I'm creating a Titans server mod that I have no intention of working with the vanilla game.
@dom314 said that shadowing ex1 doesn't work. https://forums.uberent.com/threads/how-do-i-shadow-pa_ex1-files.69697/ In most cases you'll just shadow as if the files were in /pa It functions much like a regular mod, and I'm surprised it wasn't /ex1/pa, which would have opened things up for some tactical UI overrides without extra effort.
It's a bit unfortunate, because this effectively means that I can't make a mod that is compatible with both versions of PA.