I am an industrial photographer and have shot handheld with tech cams for many years, initially with a P25 on a Horseman Sw-D, later with the same back on an SW-DII, and these days with P45+, P65+, Aptus12 or IQ180 on an ALPA STC.
The Alpa STC has been a godsend for handheld work. It is light, uncomplicated, very robust and fast to work with. My small hands sometimes struggle to reach the shutter release on the lens, but the Alpa Sync release puts the shutter release in a better position for my hands. I often also use a short cable release and trigger with either my left or right thumb. I find I get less camera shake when working at slower shutter speeds (I often handhold as low as 1/15th sec).
You should have no reservations about having the camera around your neck. My Alpa has been snow trudging in the Alps, clambering on scaffolding on highrise towers 85 floors up in Dubai (in 50deg C heat) and just last week, on an oil pipeline project in Southern Iraq (hot, dusty and swampy area South of Basra), whilst hanging from my shoulder. The camera is very robust.
For the sake of the comparison, I did the Iraq shoot with both the Phase One DF camera with 28mm Phase lens and the Alpa with the SK35XL, shooting mostly handheld. I found the DF and 28m almost unbearably cumbersome and had a tendency to swing around a lot when clambering over machinery. The Alpa tends to stay against my body better in these situations rather than swinging around and baging against things. The other striking thing was that I had a much higher number of usable frames from the Alpa than the DF. Even allowing for the fact the SK35XL is optically better than the Phase 28 in every way, there were a lot of shots scrapped from the DF due to camera shake. My guess is that the weight of the DF being harder to hold, combined with mirror-slap was disturbing stability. The Alpa being lighter and not having a mirror was superior by a large margin for handheld work of this kind. (I was shooting mostly around 1/30th at f8 with both cameras).
I always shoot LCC frames. It's pretty easy to pop one frame at the shift setting and aperture you've just shot a string of images at. I carry my LCC filter in a thigh pocket on its own so it doesn't get damaged by abrading against other stuff. My current filter has had 4 years of heavy use without damage. It is obviously important that your LCC filter remains unblemished.
Since I've started shooting with the IQ180, I hardly ever use my viewfinder. I sight over the top of the camera and confirm with the awesome screen. I only use liveview when working off a tripod at less than hyperfocal distances, and occasionally when I want to shoot wide open and have DOF issues. This combo of the IQ series backs and the light, robust TC or STC really is the killer kit for wideangle handholding in MF. The ergonomics of these two little Alpa's really make them ideal for fast, stable and comfortable handheld shooting.
Cheers,