Has this been done before? My idea is that the DSLR lights up its green focusing light to be a sorts of focusing aid when manual focusing. I'm usually getting 70% accuracy during daytime, 30% accuracy during low-light, and you can just forget accuracy during no light. I know there's such thing as a distance scale, but I'm not good at it.
I doubt Pentax would have any troubles putting a trigger to turn the light on: It has the DoF preview, Raw/FX, green button, and even better, the AF button.
Now for turning the light off: you could hold the trigger until it is in focus, then you let it go. You could do a toggle. Or it could automatically turn off during a half press. (Or delay the shutter by a millisecond)
Actually, now that I mentioned that, why not turn it on during a half-press and then off when the shutter button's fully pressed? Kinda like what happens when you autofocus, but you're the AI.
Also, since there's a better contrast between the foreground and background, I guess the AF confirm will work even better?
I'm saying this because I buy manual lenses (tight budget but awesome lenses), and it would be nice if I could manually focus better. And since Pentaxians love old manual lenses, I think this would be a great contribution to the love.