Our monk wanted to trip someone a few sessions ago using a monk-ish-punch/kick. I said, "roll your attack, and that becomes the DC for the Dex save the guy gets." I guess my point is, being a little creative and open ended with the rules allows for customization of just about anything you can think of.
Nice ad-lib, that's very close to the official rule ("Shove" is Athletics vs Athletics/Acrobatics to knock the target prone or push them 5').