Stomp seems to be the phrase of choice inhttps://gist.github.com/1wErt3r/4048722
(a commented disassembly of Super Mario Brothers. Warning might slow your browser down somewhat loading that).
Line 11477 is probably where I would look it deals with speed/height after a stomp. Whether you can put a different number in there and get something fun going on (it is FD there, which if you are going to be increasing 8 bit numbers does not leave a lot of room, on the other hand if it is some kind of reverse/negative affair then seems far more apt) or are going to have to play assembly hacker to actually do something I don't know..
11439 and down dealing with stomping behaviours in general.
Even without the niceties of a disassembly then player speed should not be too hard to find and potentially manipulate (it will be tied somewhere fairly directly for the on screen location after all).