Have you looked at TI's PRU at all?

Emulating the RPI PIOs instead of the TI PRUs is really a miss.

The PRUs really get a bunch right. Very specifically, the ability to broadside dump the ENTIRE register file in a single cycle from one PRU to the other is gigantic. It's the single thing that allows you to transition the data from a hard real-time domain to a soft real-time domain and enables things like the industrial Ethernet protocols or the BeagleLogic, for example.