If you can still bend your thumb, i might suggest you use a slightly modified leather glove and sew in an insert plate
with a cylinder tube, resting on the inner side of your middlefinger knuckle. Wrap the cylinder in leather to make it more comfy,
and better for string on release.
Then you just slide the string between your hand and the cylinder, grip with the thumb, and release by easing up on the thumb pressure.
To me, it sounds as if it should get a smooth and stable release that way, and be fairly comfortable for your hand.
Sort of a reverse thumbring.
I composed a small drawing to better explain.
plate on the inside of leather, cylinder sticking out
Sorry if the idea is rubbish, but i have never thought of this before..
In any case good luck
-SveinD, Norway-