FYI - I just d/l Avast! 4.6 today and was curious about your post, especially since the ISA plug-in appears to be only a Web Filter.
I didn't test for streaming over HTTP, am configured for the standard MMS port (1755).
The first few streams I clicked on downloaded the webpage with an embedded WMP, but no content streamed.
Seems a number of streams are mis-configured Server-side, won't work for at least a couple reasons (unknown, but different reasons).
Finally, I was able to view the "Hitchhiker's Guide to the Galaxy" and listen to the Ashlee Simpson stream.
In all cases, clicking on the hyperlink to launch the stream definitely caused changes to the WMP, so I never found that outbound code was blocked by the Avast! web filter (default settings) and since I'm not receiving the stream over HTTP, as expected I didn't experience any issue related to the ISA plugin.
WMP 10
ISA2K
Avast! 4.6
Tony