I once proposed a fighter launching system that was rather cumbersome, but probably doable now that things like scripting exist.
The first thing you need is a properly set up capital ship. In addition to a launch tunnel, it would also need a series of linked submodels using the animation code. Now, while subobject translations (that is an animation where the subobject moves in a straight line rather than the rotation effect we're bound to right now) are not currently possible, it can be faked by attaching hinging submodels together and have them trigger rotation animations at the same time. As an example, if you hold your arm out straight, and then pull it in to touch your shoulder, your forearm and upper arm fold together, rotating around your shoulder, wrist, and elbow for both. The result is that your hand has moved in a straight line towards your shoulder. If you can simulate this with a series of submodels, then you can effectively cause your catapult to move in a straight line towards the end of the launch tunnel. The cool thing is that the submodels, with the exception of the catapult itself, can be made invisible and non-colidable, so they have no effect on anything themselves.
The next step is to attach a dockpoint to the catapult and set the player's initial status as being docked to the catapult. The tricky thing is the timing. The swinging of said linked submodels has to be very fast to get the kind of speed you'll want to simulate launch. Also, a jettison-cargo (or a scripting equivalent) has to be timed to release the player craft from the dockpoint a microsecond before the catapult would reach the end, so it wouldn't pin the player to the catapult when it hit the end. Submodel animation code also has fields to control acceleration and deceleration (IIRC) of triggered animations so the launch would appear to speed up towards the end of the tunnel, rather than go instantly to max speed and maintain that for the full launch. Of course, the player would also need to have the controls disabled until the fighter has exited the launch tunnel.
The main reason that I support this kind of launching (for all its cumbersomeness) over simply using the sexp that nudges an object in one direction, is that the later would require that the vessel doing the launching would have to be stationary during the process. Otherwise the player would deflect off the hull during launch. Also, I seem to recall problems with that sexp when attempting to use it down the central axis of an object. It wasn't working for this.
