I'm no addon expert by any means, but this intrigues me (the fifth element tracking bullets come to mind)...
From an addon standpoint, couldn't you make a weapon that looks like a rifle (or whatever) but is config'd like an AT Launcher, fires "missiles" that have a tiny p3d model instead of a rocket, and have them lock onto the target?
I suspect that animations would be an issue though, as the "class" for launchers all use the same animation to hold an over the shoulder style launcher...
-Grendel