Pose Libraries are just Actions! (*)
(*) The fine print:
- The pose data is just stored as keyframes in these Actions
- The "secret sauce" is that Pose Library actions use markers (stored inside the action) to tell Blender which frame each "pose" is stored on. The name of each marker is the name of the pose. These markers are drawn as the taller, "bone" shaped markers.
- If you're going to do anything to these Actions, make sure you keep the markers and keyframes in sync!
Knowing this, if you want to:
A) Relink Pose Libraries => Do whatever you need to do with Actions to do so.
B) Reorder Poses => Select the marker AND keyframes (remember to select both, or things will go bad) that you wish to reorder, and move this to the very end (out of the way). Then, reshuffle all the other markers + keyframes (taking care to remember to move each marker-keyframe pair at the same time) to make room. Finally, you move the pose back in, in it's new home.
C) Use an existing Action as a Pose Library => Just enable the Markers -> Show Pose Markers toggle, then create a marker on each frame you want to be able to reference via the Pose Library. Then, just start assigning this action the Pose Library slot to use it!
As an aside, from time to time, I do get asked when we're going to get some more improvements to the Pose Library functionality. As I've signposted in the animation system roadmap, improvements such as blending between the stored pose and the current pose, thumbnails/swatches via the "brush selector gallery", a cleaned up API + outliner support, and reordering functionality are on my list of things to work on here. However, so far these efforts seem to be somewhat cursed! Everytime I've tried to look into the pose blending feature, life has found a way to suddenly dump several weeks-months worth of work on me, leaving little to no time for Blender dev anymore for a while. Argh! But hopefully, we'll get there eventually ;)