I wrote:

I would like to find the source directory of an installed ASDF system even if it's not loaded and even if some of its dependencies are not installed (so it couldn't be loaded anyway). It seems that I can't use SYSTEM-SOURCE-DIRECTORY in such a case. What should I do?

Replying to myself (I often do that when I'm alone :-D) it seems that LOCATE-SYSTEM could do the job...