Don't wait for a Pentax FF and don't wait for a Canon pancake anymore, I just opted to Do It Myself and Immediately! :-D
Here is what I've done. Read thoroughly if you want to follow! And do anything at *your own* risk just in case!
Well, first of all, you need to have a FF pancake, like this, actually the thinnest and lightest one in the world and ever!
And of course you need a screwdriver with fit size.
Remove the three screws around the mount, note that the one shown in the photo below is fixed and cannot be removed (and should avoid try to do so):-
Note that I used the larger magnetised head screwdriver you can see in the above to pick up the really tiny screws and avoid dropping them, which should be made of steel.
Pick up the cover and you can see the inside:-
Next remove the aperture lever by unscrewing the two black screws which hold it:-
Keep properly all the detached parts especially those really tiny screws before going on to do anything else!
Put on the adaptor. Turn the lens to focus to the "rearest", i.e., at the farmost "infinity". Double fold a small piece of lens paper (at about the same size of the rear glass element) and then use some tape to fix it, in such a way that the tape and the paper cover is as close as possible to the lens, by all means:-
Put the whole thing onto a (Canon) FF body. In this case, it is my 5D3:-
Point the camera to a light source and try fire the camera for the first time! Although it is somehow blind now (with the smallest aperture of the DA lens and the double-folded lens paper in between!), the camera should still be able to see something! Here are the results:-
There was no hitting of the moving mirror to the rear element of the lens and there was no error message displayed on the body and most importantly, the pictures were successfully taken for the *whole* frame as seen! :-D
After all, the last thing to do is to open up the aperture of the DA lens. Just roll up some lens paper and insert it into the space at the appropriate position as shown below. Adjust the size of the roll and push/pull to get the desired aperture size (as the paper roll is somehow flexible and the spring action will keep it fixed)!
I opted to close down/open up the aperture blades by half of the total diameter of the lens and as such it is now two stops stepped down from maximum, i.e., at f/5.6.
Here are the first results. Click to Enlarge in New Tab:-
(Full EXIF Preserved)
Well, they are incredibly sharp and detailed (except at the farthest extreme image corners)! And, I am sure that the results are far better than when the same 40XS lens was put on a K-5! That is, the lens actually has more resolution to deliver on Full Frame! Not even to mention that a 40mm FF lens is now 40mm, as it should be! (But not 60mm something effectively! :-()
Btw, the weather is overcast and I shall try to shoot some better samples on a bright day later on!
Related:-
When the Super Pancake Meets Different Bodies~
Compatibility List of Some Pentax Full Frame Lenses on Canon 5D Body
Sample Photos of Full Frame Fisheye and 43 Limited on 5D
Preview: Cantax K5D Full Frame! ;-)