I've quite enjoyed this thread because I too have tried to use my SWC with a hand grip, so your experience is edifying. One question regarding the mounting plate you had made: what is preventing the entire mass of the camera from "unscrewing" itself off the hold-down screw that comes up to the mounting plate from the grip handle? That was one problem I encountered -- that the shear bulk of the camera + lens could cause it to begin to loosen the friction-based connection between the grip and the body.
Perhaps some Loctite Red 271 or high temperature 272?.
I'll also mention, for the benefit of non gun owners, that the pistol grip of the Hasselblad flash, and shutter release is very, very like many Match pistol grips, including Olympic and large, International Competitions.
It would not be much beyond the carving skills of a good gunsmith of custom pistol grips, to make beautiful 'Hasselblad' pistol grips in Left or Right hand versions, with you of your camera service attaching a hot shoe, mounting stage and shutter release, perhaps out of beautiful burl or splatted Maple, Mahogany, Ebony, Teak or White Oak.
If you want a similar set-up as the OP has created with his grip, but in wood take a little DuckDuckGo search for images of pistol grips.
Cheers