I know this is a bass ackwards way to do something but here it goes
say you want XXX movies but don't want everyone to be able to see them...eg: kids
simply rename the filetype to .xxx And then make a special player for that file type, adding it to only the accounts you want to play that file type
by this method it still sees the file as a "avi mpg mpeg mp4 m4v mkv mov wmv ogv divx m2ts" and converts to flv or whatever file type you convert to now...
I do this for xxx for a few people and don't see any issues with other "levels" of security if needed.
like for say .pg13 file type or something along those lines