In particular, I've transformed the previous experiment into a useable system that supports more than one corner (now almost unlimited) and objects such as the small blocks in the screenshot. The shadow (and the system) is far from perfect, but it should work fine for my sidescroller. On the downside, I did not manage to make it a shader model 2.0 effect like the rest of my game's shaders. As this is supposed to be a gameplay element in 2-3 levels, this alienates 10% of possible gamers (according to a recent Hardware Survey by Steam).
"Falls das Resultat nicht einfach nur dermassen gut aussieht, sollten Sie nochmal von vorn anfangen..." - Manual