IB has done well by me the couple of times that I needed to ask for a bust, as well as a couple dozen times that the exchanges or contra-party wanted to bust a trade. Even when the exchange had already made a decision, and I explained how that would be grossly unfair to me because I had already laid it off, IB went back and got them to change their mind and adjust the price to what was fair for both the contra-party and myself, instead of busting the trade and leaving me with a position.
While I never thought I'd defend Instinet for anything, my best experiences in this regard have been with them (via IB). They seem to understand the issues facing traders, particularly in illiquid stocks or times of day, and will tend to adjust a trade instead of busting it if you let them know that you already laid it off. While you might not get to keep the 10-point windfall that you thought you'd get, you'll usually get adjusted to a reasonable price, in my experience.
While it might seem stupid, on the rare occasion that I think the contra-party made a mistake, and the price is in kind of a borderline area, I've called IB and asked them to ask the ECN how they will rule if the contra-party asks to bust it. If they say it's OK, I suppose they could still change their mind later, but they haven't in the past. I can then consider it mine and deal with it as such.
In my experience, I've never been able to keep anything more than about 10% away from a "fair" price. If you get one of these, you'll probably want to consider hanging onto it if you can't lay it off at a reasonable price. There's always the risk that the guy that hit you was right, and nobody else knows about it yet. Learning to "arb" this decision is one of the things that only experience can teach.