The scenario is that I have a (old) back level version of local-time which I have registered as a local project. Of course, this clashes with something else I want to do, so I want to unregister it as a local project (but not remove it) and use the quicklisp version. Any hints on how to do that would be appreciated.
There isn't a short way to do that with Quicklisp right now. The easiest thing is to move it out of ~/quicklisp/local-projects/ or your ASDF source registry directories.
Quicklisp works by modifying asdf:*system-definition-search-functions*. It could be feasible to add another function to the front that loaded certain systems from Quicklisp regardless of ASDF or local-projects visibility. Nothing like that exists today, though.