It would be nice to see this package in the EPEL 6 repository. My interest for this comes from the fact that it is a dependency of XBMC. I've managed to build libass-0.9.12-1.fc15.src.rpm on EL with no changes so the amount of work for maintaining this package in EPEL 6 should be very minimal.
Here is some information about EPEL to help you get started:
I'm already using my locally built version of libass-0.9.12-1 with XBMC on EL 6 and it's working fine.
libass-0.10.0-1.el6 has been submitted as an update for Fedora EPEL 6.
libass-0.10.0-1.el6 has been pushed to the Fedora EPEL 6 testing repository.
libass-0.10.0-1.el6 has been pushed to the Fedora EPEL 6 stable repository.