hjm,
I have a holster that might work for you. Its a little different than a shoulder holster, and will definitely work better than a shoulder holster under your shirt. Its called the "Confidant Shirt Holster"
by Kramer Leather. Its basicly a undershirt with 2 holsters attached to it. I can conceal a Gov. Model 1911 with a double reload under a dress type shirt that is tucked in. No coat is necessary. I dont use it all the time as I prefer a IWB type holster, but it does offer options, especially for larger pistols. They recommend getting one size smaller for a better fit with the weight, which is what I did (med. instead of lg.) its snug on, but the gun doesnt flop around when it's in the holster. I can see where it might tend to move around if the shirt fit loosely.
Here's the address:
http://www.gokart.net/cgi-bin/store....id=%%cart_id%%
(its on the left side down towards the bottom.)