Cheers guys.
Embarrassingly, I forgot I forgot to answer this. Simply put, the direct-inverse alignment with erg-abs marking I'm making use of is a vague riff of what happens in the rGyalrong languages, influenced by marking structures my source language (which I'd prefer to remain a bit cagey about at the moment, for sheer perversity.) I don't have a rationale per se beyond "a natlang's done something pretty damn similar!"