I love this idea, sounds like a blast.
How about writing up some background for a pirate captain with a cybernetic parrot? Please, oh please, oh please!
Anyone ever heard of the group Alestorm? I can't think of more appropriate music! Pills, Duggie, pills are good...
Seriously though, sounds like you're on to a great idea. I played a couple of pirate games recently and thoroughly enjoyed it. I know that you can take escort carriers to flesh out the attack craft a bit but one thought I had rattling around (I sent one of the trained squirrels to chase it down but it's gone MIA) was a special flagship with a complement (complement; grammar WIN) of assault boats might add some interesting flavour to a pirate fleet. Nothing crazy like the Jovian class, but a single ship of battlecruiser size could be an interesting addition to larger pirate fleets.
You could make this a special objective for the adeptus arbites - destroying/capturing it netting extra bonuses in the same vein as a space marine venerable battlebarge.
Enough random babblings for now.
Cheers,
LDTM