The early German built Voigtlander Vito series rangefinders and also the Voigtlander Ultramatic and Ultramatic-S SLR have the shutter 'switch' exactly where you describe. Combined with a between the lens Prontor LK leaf shutter and the smooth shutter switch I can shoot handheld reliably at 1/15 and with a little care at 1/8 with my Vito CLR. It is still a downward press, but I find it easier to hold the camera steady while using it than the conventional shutter releases on my other cameras.

You can read more about German Voigtlanders here: http://www.amdmacpherson.com/classiccameras/index.html