You can not directly roll a 457 into an HSA as I understand it. You *can* do a one-time rollover from IRA to HSA, though that is subject to annual contribution limits (for joint filers this is $6550 for 2014 but $7550 for age 55+). So you'd have to first go from 457 to IRA, then IRA to HSA. Given that it's a one-time deal and counts toward the contribution limits, I don't know that this is worth doing unless you don't have other funds available to "seed" an HSA.
In reality, if you have to take out a larger distribution in order to fund an HSA, it comes out as a wash in the MAGI, assuming the pension is taxable. That's because it would reduce your MAGI from the pension income but would increase by the same amount of additional 457 withdrawals to pay current expenses. Have I understood the question correctly?
"Hey, for every ten dollars, that's another hour that I have to be in the work place. That's an hour of my life. And my life is a very finite thing. I have only 'x' number of hours left before I'm dead. So how do I want to use these hours of my life? Do I want to use them just spending it on more crap and more stuff, or do I want to start getting a handle on it and using my life more intelligently?" -- Joe Dominguez (1938 - 1997)