I dunno about other labels, but with the two comps I put out, Oak Ash Thorn and Weirdlore, I didn't tell the world about them till they were ready for release. I imagine most labels take a similar approach - in my experience people running small labels tend to be quite opinionated about music and confident in their own musical taste... I had a pretty good idea who I wanted to invite to my albums and I wouldn't have wanted to try to manage a free-for-all where all and sundry were either trying to get involved or recommending their favourite artists.

The only exception that immediately springs to mind is Dark Brittania - I believe Cold Spring Records were touting for contributors. The other exception is the free CDs cover mounted on magazines, where contributors can buy their way on by way of self promotion.

I doubt there's a website listing such up and coming releases. It would require label owners getting round to notifying them, for a start off...