Update: I'm an idiot! Or at least not as smart as idrive. I normally use the birds-eye-view mode of the backup camera, but I switched it to the normal-view today and BAM, hitch-zoom-view is available and working! Not sure why it depends on that since it's a completely separate menu option, but oh well.

I'll still try to find a trailer to verify light operation, but everything appears to be working well for now with the coding I've documented here.