According to
Imaging Resource: "When in 1.3x crop mode, the camera displays an inset frame guide to indicate the smaller size image. ... It's nice in that it not only helps you with composition, but it also serves as a reminder that 1.3x crop mode is activated ... The downside I found is that the rest of the frame is still in view"
Pic related, from the D7100 brochure PDF:
In this mode, the advantage for tracking AF is apparent, in that the D7100's 51 focus points cover almost the entire shooting frame.
From these descriptions, I guess for Pentax to implement this feature in a similar way would require more than just a firmware fix. Since the K-3 viewfinder is optical, the D7100's visible 'DX-crop' framing guide and icon in the viewfinder would require hardware changes to the viewfinder overlay (unless Pentax anticipated introducing this feature at some future point, and have already accomodated it in some not-visible way).
The K-3 AF would probably have no problems working with a 'DX crop' mode though, without any alterations, since all the 27 K-3 focus points are pretty much bunched together in the frame centre, just like the D7100's.