I don't know if this is possible in Blender. And probably have been thought already. The fact is that is present in 3ds Max's Character Studio. In its module called Biped. I worked in certain job with that feature, and now I miss a lot that whichever the software I use.
It consists on you can like "glue" a joint to whatever the point (world, global, however you call it) in space. You position the joint by doing fk or ik rotations, and then , with it yet selected, hit a button or key. So, that joint is pinned. For that frame and keyframe, and for the following ones (not the previous ones) till you "release" the pinning, hitting othe button, or better, key. (maybe same one, doing a toggle for each joint)
So, a joint could be then "pinned" for a frame, a keyframe, for several frames, or several keyframes. having it available so quick, is what actually improves so much the animator's workflow. And the great freedom of ik and fk moving/roating bones while bein sure that foot (or feet) stay glued to the ground in its position, or that hand stuck to, for example, a branch while the character does balance keeping the hand in place.
This ends in having way more hours on actual animating than in fixing constantly those pivot points, which you anyway will ever set 100% fixed if there's no pin feature.
Also, really avoids the need of very complex rigs with lots of constrains, etc. Indeed, complex rigs will still benefit, but I mean I can do very complex animation, and very quick just with actual Blender bone system and a joint pinning feature.
In Character Studio 4.1 for Max 5, it is called "fixed key". Is a button you just press and press again to release at whatever the following frame or keyframe. It has a variant also useful, called "sliding key" which does something similar, but more free...is not that useful, as what is really needed is a total fixed key; though, I used for some situations where I wanted the foot too slide a bit, but not go bellow the floor, etc....
usually, sliding destroys most animations, and means a lot of work for solving that each keyframe.
Probably is possible with complex constrains conbinations, and complex rigs, but I think for the not very expert Blender users, and also for speeding workflow of animators, if it were done this quick an easy, would be great. Also, I think for 3d games export, it's better to keep all in a single armature for a character, if possible, this also helps avoiding unwanted leg (etc) extending, which is good for toon models; not so good for accurate humans animation.
And anyway, thanks a lot for at least reading this suggestion

[ I have not seen this feature anyway in the test builds of Tuhopuu nor BF-Blender in the history of changes.]