I can’t find any keyable attributes about blending between IK & FK with Mannequin’s arms or legs.
Did you find solution? I have the same question, actually
Unfortunately, I don’t use ART toolkit anymore.
But greatly thanks for your answer!
It must be useful for other people
I found this in ARTv1. If you select the “Rig_Settings” node, the channel box will show all sorts of keyable attributes, including IK and FK mode for both arms and legs.