Constraints can currently only use World Space, Local Space, Pose Space, Local with Parent.
This patch adds Custom Space with a custom object to define the space.
The motivation for this patch is the following situation.
If you have a copy location constraint in your armature you are already limited when you don't want to use a specific axis. It can be done, but its unneccessarily difficult (as far as I know). But the situation is close to impossible as soon as you have a root bone in your armature (which I was told you always want). I need to only copy the location along a custom axis for a lot of things. The same goes for copy rotation/scale and all the limit constraints because I need to make that axis relative to the root bone transform.
Here is a demo video:
This is the file of that video:
Actually this is the limiting factor for most of the things that I did so far with bones.