With my Tech V I usually use a simple Norman flash bracket to position the flash over the lens axis. The only modification necessary was to add an extra hole in the top plate of the bracket for better positioning of the flash. Normally I just use the bracket in place of the Linhof ergo grip, but if you like the grip, you can set up the bracket on the right side of the camera, or you can attach it to the tripod socket in the bed, instead of the one on the back. If I'm using the camera on the tripod with on-camera flash, I'll attach the bracket to the bed socket, and use the socket on the back for the tripod.
If I'm using my Norman setup, I just screw the flash head to the bracket using an ordinary screw instead of a knob, so that there's a little more clearance for the Linhof viewfinder. If I'm using my small shoe-mount flash, then I use a Hama flash shoe designed to screw into a flash bracket.
If you're really enthusiastic about using the flash mount on the rangefinder, there is a company that still makes an e-clip for mounting any round thing to the flash mount. Try a search at the LF forum at lfphoto.info for "e-clip" and look for a post by Bob Salomon.