Although there was not a lot I saw of the scene, I snagged a few frames of Mr. Q hanging around waiting to film & a couple of him just as he started doing his scene, where he enters the restaurant.
After I saw a couple takes of this scene being done, I left, and shortly after came across the trailers for "Garth" & "Randy Baxter" (played by former "Good Rockin' Tonight" host Terry David Mulligan).
The episode he was filming, 718, is titled "Party On, Garth" & from synopsis I have read elsewhere will feature Sam & Dean helping Garth defeat a Japanese creature called a Shojo. Only other detail I know of it comes from Terry David Mulligans tweet where he says he says DJ Qualls gets to taser him. https://twitter.com/#!/TDMulligan/statuses/171766277227036672
The return of Garth is in this weeks episode on Friday, 9 PM local time on The CW.
EDIT - you can see a few other photos from this episode (including a different set on a different day) at this blog post from this twitter user @susangittins http://yvrshoots.com/2012/03/dj-qualls-returns-to-supernatural-to-party-on-garth.html