Exception during runtime resolved:
After calling the parse()-method the underlying OneArgumentOptionHandler calls setter.addValue(value) where value is a byte[6]. This method does - with a detour over ArrayFieldSetter - something like

byte[] field = new byte[1];
mac[0] = value;

which causes an IllegalArgumentException.

I have modified my OptionHandler to take the (correct) bypass over setter.asFieldSetter().addValue(value) which solves the problem. Therefore MacAddressOptionHandler is not longer a OneArgumentOptionHandler.