Is it possible to set the value of workofs_out in a post processor to use a misc or real value?
Exactly.
The whole "auto-increment the work offset" function was created and intended for Horizontal work. It allows you to program Front > Right > Back > Left planes, and get B0 G54, B90 G55, B180 G56, and B270 G57 > automagically. But this behavior does a terrible job at "advanced horizontal programming", where you may be cutting at B0, but on the left or right part, held on the B90 or B270 face, where you want a separate work offset, so you can make adjustments to each part/f