That's correct, zLineOfAction does not plug into zRestShape. The approach you suggested should work. Some kind of pose interpolator could also work if you have an animated joint hierarchy in the sim scene as well.
This is a recurring question. It's tricky because the modelled state of the tricep has the contraction 'baked in'. Real muscles don't have the concept of anything but contraction, so it doesn't make sense to make a zFiber push/lengthen/loosen if we're using an anatomical approach.
Best bet is doing a rest shape in my opinion. Do a sim of the elbow bending from rest, duplicate the simmed result. Sculpt exactly what you want the tricep to look like. Extract out the corrective shape using the tool in the Ziva menu. Then pipe the corrected mesh into a zRestShape and trigger it using a curve length as you suggest above.
How does the rest of the sim look so far?