I would like to share my project implemented at FastVPS.ru for large scale backup system.
We support only delta generation and ONLY for 1MB block files (we are using fast-rdiff for OpenVZ ploop files).
We haven't support for "rdiff path" but original rdiff could be used for this purpose. But please be careful! fast-rdiff is using md4 algorithm and you could explicitly specify it to new rdiff:= with option: --hash=md4