Comment 5 for bug 1335323

I'm not sure what you're trying to do and how.

The obvious extension point for ASDF is the asdf/find-system:*system-definition-search-functions* — if you register a search function there (possibly at the end) that does the installation, then it will return the system object (or pathname to the system), and the cache will be primed proprely. If you don't want to permanently modify s-d-s-f, nstead of this handler-bind, you could thus bind s-d-s-f to a variant with your search function appended. Or you really don't want to modify s-d-s-f, you could override search-for-system-definition to add your magic function before the end.

I don't understand why you're not using this extension point, but assuming your problem is that the cache gets primed with nil, the solution would be to clear the appropriate cache entry with
      (unset-asdf-cache-entry `(locate-system ,name))
      (unset-asdf-cache-entry `(find-system ,name))