I wanted to give the Action constraint a try by making a simple translation/rotation animation, using a bone as the guide for the translation via the copy location constraint.
What I didnt know at the time, doing this directly after seeing the video, is that the Action constraint doesnt actually animate anything other than the transformation keyframes of the bone.
(This makes sense thinking about it, since it's only accessing the data of the bone directly not any other attached data such as constraints)
Video Link
Is there any alternative way to do this? Primarily for the sake of animating something on a single slider (ie. single key frame pair) such as the Evaluation slide as you mentioned.
Drivers maybe, though I wouldnt see how that works with the other influences/head-tail/etc. sliders that require different values?
This is an interesting test that you are trying out Pascal.
The action constraint is being overridden by the copy location constraint - which is why only the rotation is working in your final test.
From what I understand about what you are trying to do..
You want these bones to slide along the length of 2 different bones, yeah?
I don't think this type of thing will work with the action constraint because it would require secondary constraints to be animated.
I don't think this data can be captured by the action constraint.......unless you see up a bunch of mechanisms that converted this switching into a bone transformation.
So other solutions might be to set up a spline that has a control vert at each guide bone joint and then parent those verts to the bones.
Then on your other bones, clamp them to the spline with a constraint, and then drive the movement along the spline with an action constraint or a driver.
But then dealing with the rotation is another problem to solve.
This reminds me of the Treasure Chest project, that had the retractable cannon inside. (It opened like a sliding bread box)
This required A LOT of mechanism to make it easily animatable with a single control.