'scuse my lack of knowledge of *nix stuff, but the following (vague) description might work for you.

Spawn a thread or more likely fork a child process, set the (?real|effective) userid to a non-privaledged ID. Pass the file name/handle and the first word in the file to the child process (via a pipe or shared mem) and have it attempt to read that word and return yea/nay as appropriate.