I would take the play-within-a-play to be something like the play in Hamlet where Hamlet gets the traveling theater group to re-enact Claudius's murder of his father, not something totally disconnected like a separate play that just happens to be staged between acts. Mise-en-abyme might well be a term for this, but so is play-within-a-play. The advantage of the latter term is that people might understand what you're saying when you use it.