> why not use Bitcoin compact uint, which most PSBT consumers already

I totally agree with that, compact int parsing is already implemented in all bitcoin applications, wallets and libraries. Also, for certain (mb proprietary) applications I will be willing to use multi-byte keys where the first byte defines the application type and next bytes define application-specific fields.

I would suggest to set proprietary bytes to 0xF0-0xFC or to 0xE0-0xEF then (E for Enterprise?).