I have put together a proto-type that's embedded in the STS dashboard which was possible with minor changes to Mylyn. The current discovery implementation is coupled to a wizard but it can be extracted into a reusable component easily. There are some hard-coded properties such as the extension types that are Mylyn specific but that could be changed.
One thing to consider is that the discovery mechanism works on Eclipse 3.5 only. Support for Eclipse 3.4 would require back-porting the code that drives P2.