Alright, I think I figured it out.
Using a GUI unpacker I extracted the sound banks, where I could indeed find the XML for the Hololive commanders and their sound files. I recreated the folder structure in res_mods (.\res_mods\banks\OfficialMods\Hololive), used a dummy silent WEM file (found through listening in via a Foobar2k plugin instead of dealing with transcoding), copied/renamed that to the files called upon the events I suspected (see OP) and that was pretty much it.
Through this kitbashing I avoided creating a new commander sound mod, so it keeps it simple. Now I only need to manually copy that every new version I create an automation (probably through a batch file using something like this, but that is something for another day).
Just wanted to list my findings in case anyone with similar goal should ever stumble upon this here. It really was as simple as I thought, actually even simpler.