How about a shared memory setup?
Each device could be allowed a certain timeslot (10ms, 50ms, whatever is needed) to access the memory in a circular fashion.
Each device can read what it needs and write what it needs.
Each device could have a dedicated status or "what to do" byte in the memory. When any device want Another device to do something it writes to the "what to do" register for that device. Each time a device is allowed access to the memory it reads the byte and acts accordingly.

/Henrik.