The tough part comes when you're using a pack or hydration system, and you've got to work the gun around all those straps. The chest holster we showed in THE BEST DEFENSE is actually an excellent choice (it's my holster). I just got my Survival Sheath System S&W 329 shoulder holster, and I'll probably wear it when we go hiking this weekend (in snow up to my butt, I might add).
Overload, I applaud your commitment to weird guns! I have a 3-screw Blackhawk in .30 Carbine with a short barrel...noise alone is fatal to most living organisms. I love it to death. I say give that sucker a little TLC (and maybe some kind of metal finish on the frame to throttle back the galling that was endemic with those guns) and haul that beast into the woods. I think maybe a vertical shoulder holster might work the best with a long slide gun like the Automag, maybe a Bianchi X15.
Michael B