It's probably not what you want, but if you subdivide the quad enough the "seam issue" get less noticeable. Left is no subdivision, center is 10 subdivisions, right is 4 subdivisions.
If you find how to do it "properly" I'm interested.
do you want to only stretch the trapezoid or do you want actual perspective?
Yeah I know that if you subdivide the geometry the problem "kind" of goes away, and I suppose that in the idea of doing distortion effects etc one ends up subdividing the "sprite" anyway. So it's indeed more of a <I'm looking for the "proper" way to do it>, even if I suspect that the real and useful way would be just to subdivide, since in the worst and probably more common situation you'd need more geometry as control points anyway to do something useful. Yet, one should quantify if this alleged "proper way" is more expensive than just adding the extra geometry needed.
I'll keep you updated if I find the time and the solution!