Flash isn't a protocol. Certainly if they're streaming over HTTP (or TLS) it will go through a proxy (or at least it does everywhere else, I don't have OS X). What specifically are they doing that a "ssh -D" wouldn't handle?
You're correct HTTP connections over flash will use HTTP proxy settings. The problem is many sites don't stream over HTTP, they use some custom streaming protocol using raw flash sockets. And these do not respect SOCKS proxy settings.
I just remembered it's not actually custom protocols with raw sockets that are common and don't respect SOCKS settings but rather Flash's RTMP protocol.
