If you have a lots of filenames it is hard to realize why the Rename button remains disabled, because you have to browse through the list to see if there are any red filenames. Maybe the easiest solution would be to add a filter "Show only problematic filenames" or something like that. Two buttons also would be helpful to go to the prev,next problematic filename. Or add a new column which displays the status of the filename (eg: OK or Invalid, etc) and add sortable columns so the user can reorder the list to see all the problems he has.
Just saw that we have another open bug for that with some more details, so I will close this one. I think Andre Mirande is right that option 2 and 3 of his proposals should be fine. *** This bug has been marked as a duplicate of bug 14458 ***