Intriguing idea. It has lots of consequences. Some good, but not all. I'm undecided so far. An important aspect of the game has been retaliation. That will probably change, because who will you retaliate against? Then again, this fleet can hardly use voodoo, I assume, so all it can do is really to hit already lit up players. You may hire someone to light them up, of course, but then, why not have them so the plundering too?
Another thing this does, is insulate your important fleets. I may consider doing this: Place a level 10 cutter alone in the privateer fleet. Light up a target. Hit him a hundred times or so. Transfer all gold back to hand, and let the cutter be stolen.
Also, I now can attack targets of my own nationality without hostility simply by using this fleet. I may personally benefit from that (at least in the short run), but is it good for the game?
There is little you get to do incognito, though, since most actions require voodoo. There are some things, of course. I can clearly see the benefit for a player to have two separate such fleets: One which you only use for undercover stuff, and one to reap the other benefits of such fleets, but which will be easily identifiable as your fleet.
Another thing is Skirmishing. With such a fleet, a skirmisher can use big ships with less danger, since Stealing them will be harder. It's quite safe to keep a lot of gold on hand, since it's only one fleet, and it can only be lit up with Ambush cards. That's 20 attacks (with disfavor). Plus 1 attack for each 9 danger accumulated. 180 danger, and I can max be attacked 40 times. Plenty to keep a fleet of two SoLs and three cutters safe, for instance. And such a fleet will beat most trade fleets even with a SoL Escort.
Other scenarios? I'm sure I'll think of some. Maybe I already have, but don't want to share them...
Are these situations, and other consequences, desirable? That's a matter of taste, I guess. I'm undecided so far...