Similar to Brandon's post, I've had success with a jig that holds the broach colinear with the arbor press ram. Here's a simple mockup of this idea:
The square inset in the top fits over the end of the ram. The small hole on the front face is for a 1/4-20 or similar size set screw to hold the adapter onto the ram (you can also use a thumbscrew or similar fastener to mount/remove it).
The through hole in the bottom is a reamed hole that provides a very tight clearance fit around the shank of the broach. The tighter this clearance fit, the more accurate the broach alignment will be. When properly machined with tight tolerances, this adapter can be used to quickly and accurately hex broach objects in one operation (no need to start on a lathe).