I'd say chances are better of seeing Jim at a smaller event (than SDCC). I tried catching a few authors back in 2009. Had to stand in line for an hour or so and draw a ticket (winners had an "X" on it or somesuch). I didn't stand a chance.
But at other events mentioned here, it seems there's more of an opportunity to not only get an autograph and ask him a question, but to actually attend the occasional event with Jim as the honored guest. (Can't speak for all events and activities, of course.)
Aside from that, even if Jim is unable to go to SDCC, it'll still be an amazing con to attend.