A very simple feature proposal.
Many riggers and mogrpah animators are usually coders however not all character animators are. And most of the time we make rigs ourselves. And while IF->FK is relatively simple to achieve the same can't be said for FK->IK.
There are known methods of doing matching its just they revolve around math and coding heavily. So why can't there be a tool that does that in a more graphical way taking out a burden of coding? Yes there is an addon in Blender that creates an autorig and has this feature but what about custom rigs?
1) Click on a FK/IK Matching tool in Animation Tool Shelf
2 and 4) Press "FK/IK" field
3 and 5) Select all FK/IK bones in particular order and then press a button for to be remembered
6) Press "elbows" field (for some characters there can be more than one)
7) choose elbows tip of the bones for the IK arm and then press a button for to be remembered
8) press "calculate"
9) Under FKIK Matching tool 1 description and 2 button appear, buttons for matching IF->FK and FK->IK. And a description higher than buttons to distinguish what for this switch is. A user will be able to change the description to a meaningful one.
It can be an addon and not necessarily a hard coded feature.
While typing i also though you could create a slider for switching between FK and IK as well.
And while thinking further you could create an easy gui way for us to create and place buttons/sliders/etc and have a way to connect them to properties we're changing. But i guess this is a big separate feature.