Based upon the AK5 and SCAR design it looks like the lower requires a drop in support/boss for the bolt catch. I wonder if FN simply didn't have these details worked out for the cast aluminum FNC lower.
I have never seen exactly how the BHO works on the AK5. Do you think it's possible to develop a kit for this for the FNC? I know TSC did this conversion a while back but I don't know how complex it was to put in place. One thread on it here: https://www.ar15.com/forums/armory/New_FNC_Conversions____TSC_Machine/24-419126/