Frank Spalteholz

That's pretty nice william! Would it be possible in theory to store the pre-anim and add it as a copy of animated joints to the orig-rig and using the local-matrix of each control so that the animator could still animate on top?

Maybe you'll have good starting point by checking the worm-rig. I'v learned a lot from this example since it has also a custom viewer-state for creating sub-ctrls dynamically which is only possible in H21 (without writing your custom plugin like in Maya). So in theory you could let the animator decide how many micro-controls she/he wants to use on runtime(!). It's amazing!
I'd be really interessted in your poc if you can/want to keep sharing your progress since I'm currently working on something similar. Good luck. Apex rocks

Thank you william! I will look at this asap. in the meantime i was able to (almost) fully reproduce a prototype for a custom apex node for creating an ikfk system with auto switch-function using a custom callback. it takes 4 joints as input with tags (a root, upperlimb, lowerlimb, tip) and it follows more or less the exact same pattern like in the autorigbuilder. if you look at the code i've tried to comment all parts for your own customization. i was also leaving a graph-node below the apex-node if you want to build stuff on top manually. there is still one restriction though. The autorigbuilder creates a way more flexible system for adding callbacks by using a dictionary that automatically registers the specific controls for switching. my callback is hard-coded. if you need to change names (which is quite likely) follow my description in the file. i've learned a lot from it so i hope it might be useful for you else well. cheers