Dear Tobias, I personally think this "weakly-depends-on" is a horrible mess. If you want FOO, require FOO. If you want FOO+READTABLE, require FOO+READTABLE. And so have two systems FOO and FOO+READTABLE. I think that's what the dwim.hu guys now do. It also works better with XCVB, this way.
I'd think ASDF should include a ./configure step (there are extension for that kind of thing out there), and should then save configuration choices persistently, and check for these when loading a system.
I think this can be external to ASDF itself. At ITA, we have scripts that do things like that. My, it's ugly. Trying to get rid of it. [ François-René ÐVB Rideau | Reflection&Cybernethics | http://fare.tunes.org ] Mathematics is the Queen of Science but she isn't very Pure; she keeps having babies by handsome young upstarts and various frog princes. — Donald Kingsbury (In "psychohistorical crisis", 2001)