You're right that sliders are similar to transform constraints. Sliders map a bone value to animation frames.
I didn't quite understand this part:
The current mix of slider constraints is used to control the mixing of animations, not the mixing between bones and timeline maps
The slider mix is between the current pose and the pose from the slider's animation. It is only sort of mixing animations. It's better to think of sliders differently from animations. You don't really want to use sliders to mix animations. For that you should use the Preview view (AnimationState at runtime). For example, if your character is holding a burning torch, you wouldn't use a slider. You would play an animation so the torch burning animation is always playing.
Sliders sort of abuse an animation to allow you to apply a pose. Sliders can sort of "play" and mix animations, but it doesn't have the control you get with AnimationState. Sliders are best for storing poses and rigging that needs poses. There are many uses though, I'm sure you guys will come up with crazy new stuff!
We could have a match button, though it would only be able to set the bone from minimum value. The only problem is there's not a great place in the UI to locate it. 🤔
Was there another suggestion you had beside the match button?