The file:// protocol has always been valid for p2 repositories, and does work both for installation in IDE or in build-tools.
If your "eclipse-installation" is a provisioned product, it's not possible to use it as installation source since AFAIK a product doesn't expose its content as a p2 repository, and p2 isn't really able to read the product or its profile as a repo from it. There is no fundamental limitation to prevent anyone from implementing that, it would probably even be relatively simple to implement this in p2 (reading a product profile as a p2 repo). Once it's in p2, Tycho would automatically inherit that capability. If you find or open any issue on this topic, please share it here.