Not all El Pollo Loco locations are franchised, and of the ones that are, not all are owned by a Prop 8 supporter, but this blog lists those which are:

The franchise owner of the following El Pollo Locos in California donated money in support of Proposition 8. As one of my stand-by restaurants for any occasion, I will be boycotting the following locations owned by WKS Restaurant Corporation, owned by Roland Spongberg.

Also — If anyone out there knows someone at either CSULA or CSULB, there are Pollo Locos on those campuses owned by WKS. The campus communities should be made aware.