Dear Ken,
sorry for the late reply.
I understand from your first post that you aim to score the LET in a specific region of the VOXEL geometry you are importing in FLUKA using USRYIELD. To do so, you would like to cut the specific region with a plane such that you can define the two neighbour regions needed for the USRYIELD card.
As the idea is correct, the problem is that at the moment you cannot make intersection/subtraction operations between the voxel body and other bodies, as reported also in the past link. Therefore you cannot cut your voxel region with a plane.
Other approaches in scoring LET relies on the use of the combination of USRBIN results with fluscw.f routine. You can find a description of this use in link1 and link2. This might help you to have a first estimate of the LET in the region you are interested.
Hope this help,
Giuseppe