This really is an annoying fault in the programming.

I do a lot of animations and find that when posing keyframes where I want a character to move his body but keep the feet where they were, I simply save the file, turn IK chains on and pose THAT SPECIFIC KEYFRAME, save the pose, and then reload the file I saved before turning IK on and then applying the newly saved pose to the keyframe.

Even this doesn't always work as Poser seems to always try to apply the most illogical interpolations it can dream up, but it usually works pretty well.  A few tweeks is all that's needed.