I would have thought our letterbox memory correspondence might be a big clue here. You could leave "messages" like you use semaphores in an RTOS in the memory for the other CPU to wait for or act on or whatever.

You COULD always move over to a PC104 type PC card, there's even a company in Bristol who specialise in them.

Horses for courses, you might be pushing '52 to far, expecially if there is a reasonable amount of HMI to do as well.

If you don't have a commercial outlet for this let me know and I will put you in touch with the folks I designed my laser controller for.

Steve