There is also a sheath that Ernie Emerson demo'ed that mounts on a Denny Chalker rig and fast-deploys via wave-shaped feature on the draw. You probably don't want to don a Chalker rig under your suit but a variant of this method would be easy to rig up under a suit coat. There are videos of this demo at Emerson Knives Inc. and elsewhere.
Oh my...found this: https://www.youtube.com/watch?v=qY-3IC2NwQY Makes me question my Googling ability... Thanks!
Edit: Well damn...looks like they've been out of production for years.