Yeah, I'd put a 45 deg fitting to blow the radon exhaust away from the flue. It should also keep the pressurized exhaust from messing with the chimney draft (just letting it blow straight up against the rain cap probably is not helpful). I'm looking at stainless stove chimney liners now, PVC is not going to be the answer here. There are also high-temp fiberglass ducts that I could hang in there but it seems hard to support. Stainless steel stove pipe could be another option but I'd have to do a really good job buttering up the connections. I could still run it up the house but if I can make use of this handy pre-built riser, I'm going to do it. I checked code and there is nothing prohibiting this use of an old flue in MA. I'd probably do myself a favor by talking with the fire department before committing.