I'd imagine it would be easy enough to accomplish: push shutter button, camera closes aperture, pauses to meter, then flips up the mirror and takes the shot.
The main problem I see is the lag during metering before it lifts the mirror and takes the picture. People complain enough about P-TTL delay where it's basically doing the same thing (any P-TTL, not just on the K-3). I'm sure they would complain about this lag too. You can't nitpick flaws in a feature that doesn't exist though!
I'm sure the engineers weighed the pros and cons of adding extra functionality, and since this only applies to old lenses I can see why they might not want to spend the time developing it further. (Especially since people who are using these old lenses probably know enough to work around it or aren't using automatic settings anyway.) As much as I would like M lenses to work like A lenses, I understand that would be an added bonus feature, not something that should be expected.
I do agree that this would be nice feature you could enable as an advanced option though. (Just like the custom setting to allow using lenses without an aperture ring set to "A")