There's a Jimmy Webb song called "The Moon's A Harsh Mistress", but for the life of me I can't imagine Dennis singing that, and in any case it was on Webb's 1977 album El Mirage.
Exactly right. I always figured Webb took the title from the Robert Heinlein sci-fi book of the same name, where, as I recall, the term "there ain't no such thing as a free lunch" appeared as the title of one of the chapters as "T.A.N.S.T.A.A.F.L.". Or perhaps that's from Stranger In A Strange Land. No matter, Dennis singing it would be intriguing to say the least. There is a slight BB connection in any event, Glen Campbell having sung it in concert decades ago (don't know if he recorded it otherwise) as part of his Jimmy Webb mini-set to complement his Beach Boys medley. It's on a London concert album from the late 70's.