Thanks to Frank (aka Fengwei), who has carried out this test with the lens mounted on his MZ-S with film a bit earlier, we could know that this XS lens is actually completely full frame compatible!
http://www.pentaxforums.com/forums/pentax-slr-lens-discussion/178336-da40xs-lens-tested-film-mz-s-da40xs-kodak-gold-200-a.html
I think he has done a good job which is beneficial to the Pentax community. After all, I consider his this quick test to be useful and valuable since no one has checked this and published the result before.
Furthermore, I would like to take this chance to make an apology to Frank as the last time I was mistaken that he was a camera dealer and that he had been breaking the Pentax NDAs for times which he had agreed and signed and subsequently he felt offended for what I'd supposed. Now, provided that for what he has declared here for the last time is true, possibly he is just a super die-hard Pentax fans and keen supporter who has bought almost every Pentax new item ever released these years such that the dealer could leak every new product information to him and let him play with all those beta units. Nonetheless, I still believe that the dealer broke the signed Pentax NDA for so many times these years then. But never mind, as Frank has also pointed out, this is not the fault of him as he has never signed any agreement with Pentax, which I agree. After all, it's always good for us to learn those leaks, only if there exists people like Frank, and his "brave" dealer as well! ;-D
Okay, let's talk back about the implication of the test result, i.e., the XS lens actually covers full frame. There are two possibilities for why's that: 1. Pentax has been too lazy and/or cost conscious to re-design the lens for APS-C and they just opted to use an existing old optical design of them and just re-packaged it so as to make it smaller, lighter and thin; and/or 2. They do really plan for the Full Frame in the Future!
Besides, I have done back a quick technical comparison between the specs of DA40 Limited and that of the XS lens. It seems that both of them are 5 glass elements in 4 groups and the maximum magnification / minimum focusing distance are exactly the same. As it is now also verified that the XS lens is also having a full 135 image circle, the chance of the same optical formula is employed is actually quite high.
Last but not least, my ever-updating summary about the full frame compatibility of DA lenses has been updated in conjunction.