Is it a good idea to use a Fernco repair fitting within a finished wall cavity?
I wouldn't say its a great idea in my opinion - but on the other hand, using a coupling is generally code compliant.

I'm not a huge fan of those repair couplers, but they are usually approved for joining like materials underground if they're fully sheathed (the coupling is surrounded by stainless steel - called a mission coupling) so I don't see why it would be a problem in the basement unburied.  Whatever you do, make sure you properly cement that coupling in place.

I would call the inspector on that one, just to be sure.  Something like this is one of those plumbing code issues which varies from jurisdiction to jurisdiction.  When dealing with sanitary sewer piping, I wouldn't scrimp out on a fitting just to save a few bucks... the thought of a potential sewer joint failure in your basement is too awful to imagine.