There are some cool suggestions here.
How I'd do it is by using masks just above the neg. It'll be increasingly difficult as the size of the neg gets smaller. So it'd be best with a 5x4 neg or MF. If you were gonna do it with a 35mm neg I'd suggest going a totally different route involving scanning the neg and using just black on transparencies and print it out to size rather than doing it by hand
Mask out in thin black card the areas you want to keep sharp using an exacto knife to make a cutout. Keep both parts as you'll need them both.
Use the mask to block out the background while you are keeping your subject in sharp focus, expose as you would normally. Once you've done that, put the print into a darkbag and swap the masks around. Defocus the lens and re-expose the paper. That should give you a close approximation of a shallower depth of field.