You can, well sort of... it plays in the media library browser window, not the player. So the player volume & EQ don't affect the audio. But output plugins seem to work... I haven't tried DSP plugins. You have to add YouTube as an online service:
http://forums.winamp.com/showthread.php?t=375338
In the URL field enter:
https://www.youtube.com/?cmd=mini
...and you're good to go.