If you know the length of the file then you could use sched_api to schedule something. For example, if you know that your file is 12 seconds long you could do something like this:

sched_api +9 none uuid_broadcast <uuid> <app>::<args> bleg

Check out both of these on the wiki - there's a lot that these can do.
-MC

http://wiki.freeswitch.org/wiki/Mod_commands#uuid_broadcast
http://wiki.freeswitch.org/wiki/Mod_commands#sched_api