Assuming (what a way to start) you can come up with lines leading back up to the reservoir (try places like NAPA , Autozone, Pep Boys, etc.), you can do it in a vise. Same procedure but use a blunt wooden dowel or what have you (and muscle) to actuate the piston rod.

When you're satisfied the air is all out, leave the lines in place and install the MC on the booster. Next, find something to wedge on the brake pedal, depressing it about an inch. This will prevent loss of fluid from the ports when you replace the bleed lines with the regular hard lines. (Don't ask me how this works but I've done it for years, any time I had to open a brake line.)

With hard lines hooked up, remove the pedal block and you're ready to bleed. If you can, have someone press down on the pedal and open the MC fittings slightly (one at a time) for a brief second or two, using a rag to soak up the fluid. This is to expell any air that might have got in as the lines were exchanged.

I think the reason for bench bleeding, and what I just described, is because it's hard to force air bubbles down and out from the highest point in the system. Hope this helps,
--
Bruce Young,
940-NA (current)
'80 GLE V8 (Now gone)
'83 Turbo 245
'73 142 (98K)
'71 144 (track modified)
New 144 from '67 to '78
Used '62 122 from '63 to '67