edited for clarity:
here's how i did it.
1. follow this video to get the one man army acheivement, but make sure to steal an armored truck instead of a normal SUV
http://www.youtube.com/watch?v=pLYzWz3HBkg&feature=related
wait 5 minutes like the video says and continue down the subway until you're on the other island that still is not unlocked at ths point.
now i've only done this once so maybe i just got incredibly lucky, but i doubt it. i could tell cops were everywhere below me but i only got shot at a few times. an army helicopter came around at once, took a few shots at me and blew out a rear tire, but took off pretty quickly after. keep going into the city and be careful not to flip off the tracks because you're dead at that point.
once you're far enough into the city, turn around and start heading back home. now keep in mind it took me 3 tries to get this to work so you might need a couple tries before you get the timing down.
the goal is to escape the wanted bubble right after you've crossed the point on the bridge that triggers the initial 6 star wanted level.
keep in mind once your wanted level is gone, the subway trains kick back in and you're gonna have to dodge them. i was going full speed in the armored truck and hit a train head on, it scared the shit out of me but it didnt take the truck out of commission luckily or i'd have to do it all over again.
unfortunately you dont get an acheivement for this, but whatever. i just wanted to say it was possible.