Well, we pirate in the Sigmas because it is less defended and those same traders are on their way to Kusari, or have just left. It's the perfect place to hit Kusari shipments. If Rheinland suffers too, they should trade with Her Majesty instead.
The problem with Pirate IDs would be similar to the current one. I'd get a lot of bitching. That and the QCP are official members of the BAF, much like many Privateers, like Sir Francis Drake, reached high rank in the Royal Navy. In fact, the British made serious use of privateers in the Age of Sail, and many were official Royal Navy vessels.
Also, there'd be extra trouble with pirate IDs if entering Bretonia proper for resupply. Namely Mercenaries and Bretonians that don't read the forums moaning then firing on BAF vessels. That wouldn't be desirable at all, so we'll be sticking to the BAF ID for the moment, I think.