While I've never had to deal with routing multicast messages through and ISA firewall, I had always thought this it was an unsupported scenario. Well, I was right that the scenario is not supported, in that the firewall was never tested to confirm that there are no unexpected problems with multicast routing through an ISA firewall. However, just because something wasn't tested during the design phase doesn't mean it won't work.
That's the case for ISA firewalls and multicast routing. What's good to know is that if I ever do need to enable multicast routing through the ISA firewall, there is now guidance on how to make it work.
Just head on over to https://blogs.technet.com/isablog/archive/2009/02/04/how-to-enable-multicast-routing-with-isa-server.aspx and read Thomas Detzner's article on this subject and you'll be good to go. Note that he does say that support services might not try to fix things if they go wrong, but until things go wrong it's worth a shot.
Thomas W Shinder, M.D., MCSE
Sr. Consultant / Technical Writer