Tarantino was on several episodes of Alias. He didn't play himself, but his character was AWESOME!

Ronald D. Moore had a cameo in the last 5 minutes of the BSG finale.

Carlton Cuse provides the "Previously, on LOST" V.O. at the beginning of every episode.

Marti Noxon and David Fury were listed as co-stars in the Buffy episode "Once More With Feeling" as they both had singing roles.

Granted, none of these people play themselves (except maybe RDM), but I could list more if you want. :)

Tarantino actually also guested on Margarette Cho's sitcom ... and I believe he played an Elvis impersonator on The Golden Girls back when he was a struggling actor ...

He also hosted SNL, though it may not be fair because he's a director, too.

Yeah, Tarantino was great on Alias.

And I will just stop there to avoid a Diablo Cody rant. :)

What about Joss Whedon on Veronica Mars?

Kevin Smith on Veronica Mars as well. And do we not count Steve Martin as a screenwriter because he's a multi-hyphenate?