I can see doing 1 of 3 things...

1. Outputting something extra in the apache log to say what module is being
downloaded.  I'm not sure you can do that from php, though.

2. Keep download stats inside the module itself

3. (My favorite, but not portable) Count this as a download in the forge against
the file being downloaded.  Not sure how easy it would be to reference this, but
it's worth trying.  I could help track down the proper info.