I can't speak specifically to 700c to 650b conversion on a Riv, but I have done the switch with a Sampson Silverton racing bike that I've been riding since the late 1990s. .
Fork clearance with 700c wheels limited the rubber to 28mm. With 650b Velocity Dyad rims (21 ID, as I recall) I am running RH 38mm Loup Loups with 50mm fenders. Brakes were traded out for Tektro 559s. It works! As a reference, the bike has 43cm chain stays and estimated 73 degree ST and HT angles. I've not measured fork offset (I should do that sometime!) but can share that it was built by a local bike shop mechanic who was just getting into frame building to give me a bit more trail than the original one.
How about the ride? The diameter of the 650b x 38 tires are just 18mm less than the 700 x 28s, Though that reduces the trail a bit my immediate impression was that it's all about the fatter tires, and after logging a few hundred miles on the bike that's still my impression. I've been running the RHs around 40-42 psi, so they're really cush, which I think totally outweighs any reduced trail effect. Recently I added a front rack which settled the front end down even more. The ride is smooooth (or, dare I say, "supple"). I do ride it a bit slower than I did with the 700c wheel set but do have to say it increased the fun factor for a bike that after 20 some years I almost parted ways with.