He could appear in future JJ films with good writers.
I'm a HUGE TOS fan but at this point, Mr. Shatner needs to move on. There's no way any cameo in a new Star Trek production (JJ Abrams or otherwise) would be anything more than ridiculous stunt casting. B&B killed the character in the ST: Generations films (and the ghost written William Shatner novels reversing that got terrible reviews - never read them myself as it all seemed just ridiculous to do.)
The only thing I might find interesting is if they got Mr. Shatner to do some voice over for maybe another animated series set in a timeframe where 'Captain Kirk' thrived - or even the MMO 'Star Trek: Online' could get him to do voice over for content relating to the Star Trek franchise's 50th anniversary (again as some sort of holographic or time travel setup again set in an era where the character thrived.
At this point, no, I don't want to see geriatric Captain Kirk. Hell, I even think the fan film brigade has overdone it . Bringing 'Sulu' and 'Chekov' into ham-fisted scripts involving untoward 'aging' crisis again (already done in the TOS Season 2 episode "The Deadly Years"); or having them live a ridiculously long time (aka DeForest Kelly's cameo in the TNG pilot in 1987, or the fan based "ST:Renegades" that has an EXTREMELY old Pavel Chekov as an Admiral in the late 24th century.)
At some point, we need to let go of these actors as these characters as they are just getting to old to believably portray them on the screen in any real capacity that works. I enjoyed them all - but I don't want to keep seeing them to the point they need to be hooked up to oxygen or using a Rascal/Hoveround scooter or a wheelchair.
At some point you have to say - I did a good job with the character; and move on, or let a younger actor and production reimagine the setting (Happened with various long running franchises like "Superman", "Batman", etc. and along with "Star Trek", even the "Star Wars" franchise is realizing this.) The 3 years of TOS was short, but between TAS, the TOS era films, games, etc. the original actors as these characters has a decent run and we got to see them in a lot of situations.
I really don't want to see Shatner's (as Kirk) literal 'last breath' on screen myself. YMMV.