I won't do it with the pmo. Cause their is no need for gravity in a side-scrolling beat-em up. But their is a need for tracking the y coordinates of everything so you can create depth.
I wish you all the best in making it. But to be honest their really isn't a lot of help out to be found on about beat-em ups.
Pan Tosser is probably right, although thinking off the top of my head, it seems like it could be done without too much trouble using PMO if you had to.
You could probably disable gravity and move the Y position up and down outside of the PMO. Then enable gravity when over a pit or while jumping. This is assuming it plays like Streets of Rage, for example. Not sure if it'd work, as I've never tried but it'd be interesting to see.