Found a bit of time to look into this. Just in case anyone comes across the Middleware::Response Invalid MAC address format on unregistered clients again. This happens when a MAC address filter (FOG Configuration -> FOG Settings -> FOG Client - Host Register -> QUICKREG PENDING MAC FILTER) is defined so that a newly registering client’s MAC matches the filter.
Just for the record, fog-client auto registration currently works (FOG 1.5.9.200 - fog-client 0.12.2).