detect mouse position on 2D mesh [keystoners unite]

im building (yet-another) projection mapping patch.
based on h.jit.homography (http://hons.at/software/) (thanks again hons)

now to make it nicer:
i would like to draw masks on the active skewed plane (currently i use jit.lcd alpha-glued on the mesh texture)
but i need to find the projected(?) position of mouse on relevant mesh to make it more intuitive.

here ya go