So, 2 things:
1) Scum should have something like Rebel Captive. They're Bounty Hunters, they should have bounties!
2) I should be able to put my bounty in my Firespray without using up my crew slot. I'm not sitting them next to me, they're downstairs in a plasteel box! I don't know what upgrade type would make sense - modification comes closest from a gameplay perspective but wouldn't really make sense on a ship designed to carry a bounty. Maybe a crew card that adds another crew slot.
I suppose ships less awesome than Firesprays should be able to take them too, even though why would you want to fly something other than a Firespray...