Here's what I'm thinking. This is a picture of the Ryca rearset mount. Notice the box section tubing. Thats the area that's between the swingarm and belt.

Here it is mounted on the bike. The shot from the brake side gives a clearer view of where the box section fits in the scheme of things, though you can make it out somewhat in the shot of the belt side.

Plan A is to just notch the bottom of the box section for chain clearance. If that doesn't look feasible after I get the rough assembly done, then Plan B is the idler sprocket. It looks like I could cut the angle off the idler's arm and then weld it to the back face of the box section so the sprocket sits low enough to hold the chain away from the rearset mount.
As for rubbing against the top of the swingarm, yeah, a delrin slider is probably the answer, though its additional thickness just about guarantees it will rub. If I hadn't already cut the belt guard tabs off the front of the swingarm, I probably could have used them to mount a block of delrin in front of the swingarm,extending upward, so it didn't increase thickness on top of the swingarm. Hmmmmm...
But I'll need to se how thing actually fit with the chain. Plan C is probably to grit my teeth and accept the fact I need to go back to belt drive.