Are you talking about the guy at the first table or the second? The guy at the second table looks like the photo of Pynchon taken by a stalker/journalist (you can find the article online).
well that's fucking irritating, didn't post my reply when i did the "quote" thing. Long and short, that's not Pynchon at the rehab place, his name is Charley Morgan and he's in the Master. Current thinking is Pynchon can be seen reflected in the window behind Phoenix and Wilson as they are seated at the kitchen table at the rock band's place. Wearing sunglasses and a wig, could very well be him.
I draw the line at Grateful Dead cartoons, fonpr.