They are apparently using his younger brother to finish his scenes. Can't remember where I read that, but they have a pretty close resemblance and with some added facial hair I could see it working. You can always use some CGI to make it work even better.
JT's actually pretty awesome. Have you seen the stuff he's done with The Lonely Island (e.g. Dick in a Box)? I might actually go see the film if he's in it.
On JT getting a role, why not. The F&F series is one I'm totaly fine with gathering all kinds of (famous) actors, bit like the Expendables film. Expands the "Hell yeah" factor