.45 wouldn't fit.
Take a look at how thin the forcing cones of 696s are and I think you'd reconsider. It looks like the L frame would require a larger barrel thread diameter, and S&W only uses one thread size per frame size.
Other wise, I like the idea. Taurus made Trackers in .45 ACP. They're the closest to what you want that I can think of. Unfortunatly, Trackers used an unusual moon clip thickness. Consequently, .45 Auto Rims won't chamber in Trackers. Also, it was hard to find the Tracker moon clips when the .45 ACPs were being made, and would likely be impossible now.