I know Craftsman used to sell an adapter for their RAS to mount a router. Not saying it was any good, never saw one myself. The also had a 1/4 collet you could mount on the outboard shaft end of the RAS motor (with the blade removed of course) and do some low-speed routing or use a surface planer attachment they offered. Neither sounds very safe of practical to me, but they had it in the accessory line up.
I have often wondered if it would be worth the trouble to find a used RAS in good condition (or convert the one I have), remove the RAS motor, and install a mounting plate for a mid-size router.
You would then have elevation, angle, and bevel control.