There are Unix-like systems for 8-bit micros which
might possibly be adapted
for PDP-8. Lunix, UZI, FUZIX...
Maybe pdp7-unix could be ported/rewritten.
I'm guessing the available memory would be the biggest issue.
C itself is going to be difficult on the PDP-8.
Perhaps the B interpreter from pdp7-unix can be be simplified further?