about all you could do would be to vary the pressure the boom puts on the mast for mast rotation. Boomless cats have a similar short traveler built into the clew of the main sail to which the upper main sheet block attaches. Since the end of the sail isn't fixed by the boom, this traveler performs the similar function of an outhaul on a boom equiped sail. I really don't see the advantage of adding this kind of traveler to a sail plan with a boom.