I have been around this forum for quite a while now, but wasn't a member because I never really had a need to post anything. Since I found lots of helpful info here, I decided to share a little hack I found out yesterday when I was repairing one of my SMC Pentax-A lenses that had stuck aperture ring.
I always thought that there is some kind of electronics or something inside A lenses, but I was wrong. There is literally nothing different from M lens as far as functionality goes - there is zero electronics. So upon examining the contacts on the mount I figured out that when lens is set to A on aperture ring it just pops out litte connector on the mount that connects the electrical circuit and that A lenses have a spot on the mount that is isolated/blank(plastic). That gave me the idea to replicate that scenario on my SMC Pentax-M lens and to my surprise I actually got my M lens working in "A" mode.
Picture of A lens mount:
Here is the trick:
You have to cover one of the contacts on the camera mount with tape(the one closest to lens release button) and insert something metal to make "A" connector(the only flat connector) come in contact with the metal of the lens mount. I personally used a piece of electrical tape and a thin metal strip just to test out my theory, but there sure are more elegant ways to do it. This way, with any manual aperture lens attached you can use A mode, just remember to have lens stopped down to smallest aperture(f22 or so) at all times and let the camera open it up as much as it needs.
I hope that was understandable, since english is not my first language, but just in case I added some photos. Hopefully some of you find this useful.