The system we use is similar to your proposal – a moveable block on the track, back to a fixed ratchet turning block then to a swivel Ronstan combined fairlead/camcleats. We have been using that since 1998 and it works a treat. It makes it easy to trim the genoa wherever you are sitting in the boat and there is less strain on the sheet for the crew as the ratchet blocks takes a fair amount of the load..
I pinched the idea from Tim & Ruth Jones.
The only drawback is that the camcleats get in the way a bit when the crew is trying to snooze on the thwart.