Why not just take a sheet of opaque (or even semi-translucent) paper same size as printing paper, place in easel, and sketch areas to be cut out for pre-flashing. After cutting, then place that mask over the paper in the easel & pre-flash? Would seem to give you most of the control as above but without an intermediate development step.
2. Make an actual scrap print, cut out the areas to be flashed, and place the print on or above the paper during the flash exposure (essentially paper plane masking). Note it is best to use RC paper for this since in addition to it being obviously easier to work with and faster to process/wash/dry, it has to be dimensionally stable paper so that "registration" at the paper plane works properly.