Yes, he showed up at the Gainax panel. In fact, he and his translator were the only ones there yesterday to represent Gainax. I recorded the whole panel on tape (minus the last few minutes, as my camera ran out of batteries).
Of course, Mahoromatic's ending was the first thing I asked about. I'll upload the whole video to my
Mahoromatic website when I get back home from San Jose, but for now, what I remember him (his translator, really) saying is that he frequently met with Nakayama during the later scripting, and they somewhat decided on the plot together. Yamaga said he had the ending in his head from the beginning, so from what I can gather, the manga's ending was originally going to be different from what it ended up being. Now I'm curious once again, but doubt I'll ever find the answer... HOWEVER, considering Nakayama originally said that "you just can't" end the anime the same way, the original ending was actually
more tragic than the final result. That's purely conjecture, though.
And earlier today I got to ask Chizu's voice actress a few questions at her panel. I mentioned that I had always felt that Chizu seemed to be the most intelligent character in the show and seems to secretly know what's going on and asked her if she felt the same way, and she agreed, saying that, while looking over the scripts, she was wondering how she could play such a smart character. Also, when asked what places would be the best to visit in Japan, she labeled a few of the best places to get snacks. I believe she also recommended to visit an Akihabara cafe, but I couldn't quite hear, and the translator didn't bother translating. I can look at the video later to make sure.