I believe federal charges could only be leveled if Zimmerman was an officer of the state or local govt. Since he was part of no group the KKK laws do not apply and the federal civil rights statute of 1866 that was used to charge the murderers of Goodman, Schwerner, and Cheney was only used because the killers had been LEOs. The 1866 law cannot be used against private citizens acting privately.
The condo HOA settled, Zimmerman was acting under community authorization, watch for precedent to be set on this basis.