[xjx]

Well, you're wrong (and I already played quite a lot with this trick, believe me!) : as you're already working in stop-down mode, it will still work.
The camera will adjust the shutter speed accordingly to the actual lens aperture and current ISO. Just like a wide open A lens would do... You just have to select the widest aperture available on the camera, whatever actual aperture you choose on the lens. Moving the aperture ring will physically change the aperture, and so will change the amount of light reaching the metering cell. Using the widest 'virtual' aperture available will ensure that the camera does not compute a different Shutter speed than what is needed.

And you will gain access to Av mode in the process, too! Just like with m42 lenses.

Only drawback comes when you shoot at small apertures, as the viewfinder will become very dark!

This is so easy it's a wonder Pentax didn't include this in a firmware update...
The algorithm is pretty simple, in fact, and could be enabled/disabled in an additional custom function...

If the A pin is not shorted (meaning you have either a PK, PK-m or m42 lens):
- then the aperture actuator should be in the "no action" position, so the lens will react as a preset lens (like m42 lenses, where the aperture ring is linker to the actual aperture).
- DoF preview (or AF button) could "open up" the lens, for ease of focusing (would only work with PK lenses, not m42, for obvious reasons).
- the P-TTL metering burst will occur with the lens already stopped down, so the reading will be accurate (just try the above trick and you'll see by yourself!).

This would add tremendous value (and perfect backward compatibility) to old lenses, so we cannot really hope for Pentax to include this in their firmwares... This would undercut new lenses sales quite a bit, IMO.

I think somehow my mind skipped the part about drilling a second hole and mounting the lens partially engaged. Yes, with this modification, M/K lens should work like m42 lens as your description. Only shorting the "A" pin would lead to the problem I mentioned.

[Marc Sabatella]

Your "ISO ISO" was meant to be read "Low ISO, right? If so, I understand.

Yes, and I've now edited the post.