Is there a way to use a roto layer as tracking data?

The right tool for tracking depends on the shot. From your explanation of the shot, I would think that Mocha would be the proper tracking tool. From there you can get position, rotation, scale, and even skew data that can be used to give you the position of the watch as it relates to the comp frame. That can be turned into 3D position and orientation data if you are good with expressions. 

It is also possible to generate a virtual rectangular surface that sticks to the surface of the watch that could be camera-tracked. Then you could have accurate 3D positions for the surface of the watch so you could add a 3D object to that surface.

Show us the shot and we will probably have a fairly good chance to give you a workflow that you can follow. Without seeing at least a frame of the shot and hearing a detailed description of the movement in the shot, it is awfully difficult to be of much help.

Seeing as you've isolated the watch, if you render this out with an alpha channel (I'd suggest pre-comp, but the rotabrush is such as resource hog), could you then track that rendered footage?  I would guess AE would get a pretty solid lock.

Alternatively, if you used a mask for the watch and tracked that, you should get all the points you need, but that's assuming the track worked (I'm guessing if rotobrush did, then mask tracking would too).

No. Anything beyond that will require you to show us a sample frame/ screenshot so we could advise on alterantives. Such stuff is typically done using geometry-based tracking in Nuke or dedicated tracking tools like SynthEyes, though, so chances are there may be no way to do this in AE if it can't be reconstructed from conventional 2D tracks or using mochaAE.

Mylenium