Thanks, peter3053 and Chromolume, but I guess I didn't make myself clear. I know what a miracle play is (was), and I understand the difference between a miracle play and a morality play. What I meant was that, in any event, it really doesn't make much sense for a circus troupe to sing about having "miracle plays to play," as that's one kind of entertainment that's never included among circus fare (as far as I know).
As for whether or not the lyric makes sense in the original show, I think it does. I assume the players are singing about their various methods and performance styles, from "magic" to "miracle plays," but that doesn't necessarily mean that the story of Pippin and Charlemagne they're about to perform is a miracle play. As peter3053 notes, it's intriguing to think of the play within the play (or the musical within the musical) as a morality play (or a morality musical), not unlike the EVERYMAN tale -- even though there are obviously some major differences, including the fact that Pippin doesn't die at the end of his story.