Well, SBCL and ECL do not have that problem, AFAIK. But TRUENAMIZE also seems to be used with files, not directories, from what I could gather during the debugging of ASDF. I would abstract those calls into a function PROBE-FILE-AND-DIRECTORY and use PROBE-FILE internally in this function. The purpose would be clear and sensible  implementations might profit from that -- maybe the terrible startup times for ASDF might be decreased this way.

Juanjo