Can I use a heavy duty rubber coupling on a wet return 1 1/2

Quick google search shows 140 max temp, and that only sporadically for Fernco coupling. What exactly do you mean by heavy duty? Do you have a particular one in mind?

Is this a temporary or permanent solution to your issue? If permanent why not just do it with pipe like it's supposed to be? I don't think I'd ever do any rubber on the piping that was permanent.

Since the temps in the return could be relatively high, the only rubber product I would be comfortable with in that application would be marine exhaust hose. I believe most of it is rated to 250, and it's meant to handle engine exhaust and hot water. The only way I'd do it is if I had some kind of mid winter failure and was using it to limp through to warmer weather.

You might get longer service from this coupler with a better grade rubber elastomer seals.

Max temperature for Buna N is 220*F, EPR, (Ethylene Propylene Rubber) is good for 300*F. and is also resistant to Amine based water treatment chemicals. Viton is good for 400*F.

Buna and Viton are not recommended for use with Amine based water treatment chemicals, although I am sure this is true beyond a certain concentration, Unfortunately, I do not what that concentration might be.