i have managed to make an external "h.homography", currently mac only, and absolutely fresh and untested (except by me).
it calculates the homography given a set of destination points (assuming the source is a square).
that gives you a perspective correct transformation of a plane – the thing you need for "keystoning".
open "PerspectiveMovie++", toggle on, read movie, drag the cornerpoints.
this is version 0.0, please test.
i will hopefully be able to post a better version (with sourcepoint input, and source-testing, and windows binary) and better help-file / description in the next days.
hungry hons (going to find something to eat now)