Outside of Tuatec’s plugin which I have not tried, there does not seem to be an accurate solution outside of snapping the IK bones to the target bones in every single animation with the Sequencer. I even attempted to make a second retargeting setup specifically to move the IK bones to the targets but it does not seem to work. My only guess at this point would be that my skeleton’s IK bones are parented to the root bone and not an explicit “IK root.” I will try update my hierarchy now. If I do not come back to this comment then it didn’t work.
This thread should definitely not be marked as “solved.”