I know KineFX doesn't really have "bones" but only "joints". By limiting the length of a bone, I mean to limit how far a joint 'A' can move away from another joint 'B'. For example, a stylized character's palm is A and the elbow is B. I'd like to allow the animator to stretch its arm, but within a limited range.

I guess it can be done with a Rig Attribute VOP after the Rig Pose? Just calculate the distance between A and B and "snap" A back to B if it's too far. But it's far from ideal. There is no GUI indicating the limit while the animator is posing it, because the viewport state comes from Rig Pose.

raincole
I guess it can be done with a Rig Attribute VOP after the Rig Pose? Just calculate the distance between A and B and "snap" A back to B if it's too far. But it's far from ideal. There is no GUI indicating the limit while the animator is posing it, because the viewport state comes from Rig Pose.

that's the same for all "constraints"
Rig Pose will simply not show you how the downstream constraints whether it is IK, lookat, parent ... will affect those transforms

I assume you can have wrapper to your HDA with added functionality for animator to see to work around current limitations
but it's far from live constraints that would be able to feed back to the controller values and limit them as its possible in some other dccs

personally I think that KineFX is not yet user friendly for traditional rigging or animation, neither is claiming to be as it's advertised mostly for motion processing, but hopefully it will just become more robust in the future

I agree with Tomas overall. I think KineFX is very exciting, but I don't think it's ready for traditional rigging/animating pipelines yet. Hopefully it'll get there over the next few releases.

That said, have you tried the Configure Joints SOP? There's a section at least as of 19.0.383 that allows you to set rotation and translation limits for individual axes of individual joints. You should be able to limit the Z-axis on the elbow (in your example) to restrict stretching to a certain range. Haven't tried it yet, but I remember reading about it. Since it's placed before the rig pose, it might work.