While I admire the ingenuity of people doing real-time with Linux, I just shake my head at the man hours wasted that they could be doing the "thing they want to do" instead of fighting Linux.
The PRUs are just the replacement for the FPGA resp. microcontroller. You have your relatively relaxed control loop running at e.g. 1kHz on the main ARM and the tightly constrained stuff on the PRUs, not much difference to running it on something external connected via SPI. Some kind of external hardware is needed anyways, doesn't make much difference including a STM32.
The problem with beaglebone is lacking GPU afaik.