[jacquesm]

Weird thought: that raspi module sells for $30 in quantity, you could easily run one process on that, and use the gpio pins to communicate with a host. One user visible process, one subsystem.

It would be reasonably affordable.

Upper limits on the number of processes you could run would be dictated by how many modules you plug in, you could make a backplane like model where you daisychain multiple backplanes for more processes.