I have owned a 39mm Zenit and some 42mm stuff. I understand that the 39mm (Zenit, not LTM) thread lenses can be used via a thread adapter on a 42mm body. This adapter is a simple ring, threaded inside at 39mm and outside at 42mmm. This suggests that the lens mounts within the 42mm thread and sits at the right distance from the film plane. If this is the case, then there is little chance of a 42mm lens sitting IN the mount for a 39mm lens, so I think that it's a non-runner unless an adapter is available with some optical correction.
There is, however, a chance that a 42mm could be used on an LTM RF camera as the 42mm lens would have to mount further away from the film plane, as it does on the SLR. There will be no coupling to the RF within the camera, so may not be a great help.