Not having access to the source I couldn't begin to say, but I suspect it isn't a matter 'can't do it', but a matter of 'how to do it and not break every existing figure out there'. The backwards compatibility that they have maintained has been a big feature of Poser for decades, as it has avoided the who propietary format kerfuffle. A full IK/FK solver would likely push some data into the cr2 that might explode its size. There would have to be changes in the animation data storage, as FK would permit you to designate kinematic 'pins' all along the chain that would act as the temporary 'end' of the chain.

I've long suspected that it would take branching the code in a way that you have a 'classic' mode that preserved the backwards compatibility, and an 'advanced' mode for new figures and content. Think about a new figure with full IK/FK and softbody set up out of the box (with a more traditional version for those who want hard bodies, as it were).

There's always been the Poser geeks who get off on finding and exploiting the bugs in the code (conforming clothing, anyone?), and the ones who just want things to function with little to no work involved under the hood. As long as the file formats remain consistent, I think such a branching would actually work. There would be a hard break between the old content and the new. The program would still function properly with the old, while the new would have a clean slate. The trick would be massaging the program so that it is the common element, and you could switch between the two modes. If the two levels of figures could work together, that would be the perfect outcome. Big if, I know.