Good idea. It would also give those who really don't want to deal with PvP an easier time of hiding outright, if they opt for the Go-To-Some-Distant-System route. A pirate or bounty hunter will only be as interested in them as they would any NPC, minimizing the chance of discovery, and all but eliminating the chance of outright camping, unless the trader says something to break the NPC illusion of course. xP Bravo. ^_^ Maybe an optional tag for the players that want to DECLARE they're players so everyone can see, so those with a 'Come At Me, Bro!' attitude can continue it.
Edit: That being said, while I do think this would be the best way to implement a Solo-less game, it's more like 'As Close To Perfect As Can Be' than actually Perfect. As has been pointed out, those not interested in player-interaction aren't going to be raring for 'It's LIKE Solo Mode Sorta!', and can't help but think more than a few PvP centric players would be displeased at the thought that they CAN'T identify players who don't want to be identified, leaving it up to a coin toss on whether they're targeting a player, or another NPC.