What if you were to run BGP between the internet router and the Frame Relay router through the firewall?? No need for full routing tables, just send in a default route. Perhaps the internet router would conditionally advertise this default route to the inside network based on some prefixes from the ISP existing in its BGP table.
Configure the Frame Relay router to prefer the default route it recieves via BGP, while it also has a default route it recieves from another routing protocol over the Frame Relay link to use as when the BGP default route goes away.
Just tossing out some ideas.....
Regards,
Brad H.