I can't see Pentax driving a full frame 645 CMOS 40 x 55 chip, all their current modern glass is designed around the 1:3 cropped sensor as I understand it.
I also think it quite possible that Pentax will stay with the relatively less expensive 44x33mm format, but you are a somewhat wrong about its 645 lenses: looking at the 17 "645" lenses listed at
http://www.us.ricoh-imaging.com/camera-lenses- 13 are designated FA 645 or FA* 645 which I believe are "legacy" film era models, so covering the full 70mm image circle obviously,
- 2 are designated D FA 645, which are newer models "designed for digital" but also designed for use with 645 film camera and so covering that image circle, and
- 2 are designated DA 645, designed exclusively for digital cameras in 44x33mm format.
So perhaps the fairer conclusion is that Pentax has simply not done a lot of new "digital era" MF lens designs at all, so has not greatly committed itself to one digital format or another.
It probably comes down to how good the older film era lenses are:
- If a good portion of those "FA" lenses are good enough to compete when used with a more expensive "645 full frame" model, the Pentax system is ready;
- If instead not many of them are, leaving mostly just the two "D FA" lenses worth using with a larger sensor, then Ricoh is probably not going to make the massive investment in new lens designs needed to chase Phase One and Hasselblad into that larger format realm.