What if you're a bit cheeky, mount eg your Drive storage on a VM via one of the various 3rd-party filesystem-mounting systems out there, then run $backup_client on the VM pointed at the Drive local mountpoint?

Plenty of uncomfortable chances for failure, but a few heaped handfuls of error reporting would probably be mitigation enough.

Getting the initial clone done might be a little fun, you might need to actually download the Drive folder structure to somewhere spacious so you don't run into transfer ratelimiting for the initial upload.