set-object-relative-x would be easy.
Doing it with acceleration and stuff like you're saying would be a bit harder, but would be flexible later on.
Basically you'd have to create (Well, probably I'd have to create) a forced-movement system using the camera code algorithms. Probably as simple as adding a variable to objects, creating another Camera array, iterating that through every frame and overriding an object's movement so long as the forced movement hasn't been done yet.
Throw in a SEXP to add the object to the list and set the object's variable and that'd give you mostly what you're asking.
That'd be the simplest method, but it wouldn't give you exactly what you're asking. Instead you'd get a set-object-position function in the style of a set-camera-position function. (Or since that's not implemented yet, set-ship-rotation/set-camera-rotation)
Right now the SEXP just changes the current speed. gradual movement would require changes to either the object's phyiscs info, or overriding the object's phyiscs info. Either way you'd need a separate set of arrays and checks per frame.
