Twitch.tv does exactly that for their non-Flash streaming (MPEG2 .ts files delivered in pieces).
Twitch definitely offers HLS as well; it makes sense to support the current generation of devices, but re-use the tech stack for future standards. Clever peeps.
For a lot of codec related items support is dependent on the underlying OS, and I don't think Windows ships with a general MPEG-2 licence.
Twitch.tv does exactly that for their non-Flash streaming (MPEG2 .ts files delivered in pieces).