So this is actually how I started this before changing to the field type approach.
Create some sort of HUD (look at Anubarak/craft-relabel plugin) in the fieldlayoutdesigner that'll give you a conditions builder based on the current fields in that fieldlayout (or perhaps just that tab)
This can then be saved as a bahavior or something on the actual field.
…
Profit!
No idea how to hook any of that up, the fieldlayoutdesigner javascript is a minefield. I hate javascript.
So this is actually how I started this before changing to the field type approach.
No idea how to hook any of that up, the fieldlayoutdesigner javascript is a minefield. I hate javascript.