It does seem that the idiom deserves a name, though, considering
how often I've come across it.

The name for this is clearly mconcat, we just have to indicate which mconcat it is: the mconcat a la Endofunctors and Kleisli... whatevers, respectively. It seems like this sort of thing should be handled by the "newtype" package, but I can't seem to wrangle it into submission.