I didn't get into vB until after the 3.5 series, so I'm just guessing here. But I don't think vB distinguishes file types, other than images, in vB attachments. Like I said, I'm guessing, but it may be a limitation with vB itself. That's the only reason I can think of that it hasn't been done since vB 3.0.
It's too bad that thread no longer exists (at least, the mod in the thread is no longer online) as it might actually work.
Although earlier today I got "Goldbrick" (mod) working with attachments. The only problem being it's trying to load quicktime to play MPGs instead of Mediaplayer. Grr.
I would *PAY* for simple all-in-one embedded media player if it would work with YouTube as well as attachments.