This is a great post/topic, though this only covers losses that are known within this forum and does not cover those
outside this hub, so in the end, OP will still have a hard time getting a repository of lost/inaccessible bitcoins forever.
@OP, there aren't any publicly known database for lost bitcoins anywhere in the web. Rough estimates point it @ 1-2 million bitcoins lost, though without any real and credible source backing the number up, it'll be hard to rely on the numbers you may produce on your research. Anyway, best of luck still on your work.
I think the idea will be just to get an estimate, a rough idea or at least something to begin with, Since there will be no way to tell even if some bit coin is lying in an address with lost keys/password.
I also think the mentioned thread is the best OP could get, since it is about "known" lost coins, he could well refer to the forum as his case study.