Bill Tindall, E.Tn.
The answer is so obvious I wonder if we are talking about the same need.

Pin board in vice; tail board held in registration over the pin board to enable transfer of tail profile to pin board.

I have seen many do this operation and all simply push down on the tail board with one hand and mark with the other. I have never had any problem holding these boards in registration while marking. The downward force on the tail board is substantial compared to the piddly force from the marking knife that could move the boards out of registration.

I'm all for training wheels if they help getting started. Some could find clamps useful for this operation or think they could need clamps for this operation. I predict the clamps will become vestigial after a few drawers are made.

The place to add an enhancement is some sort of fence to quickly positioning and assure the base of these two boards are in the same plane when marking.