Howard, great pics and explanation! I can see that being harder to do with a stiffer/leather holster but if its one of these
http://www.unclemikes.com/products/i..._holsters.html
(if i posted it right)
I can definitely see it possible using the actions that you describe. Seeing in the above link how the Glock's trigger area is exposed and neoprene being being semi soft i can easily see a thumb or a finger getting in there.