On the 2000 Civic Si, I've got a couple of bolts that have rusted themselves into the metal sleeve in the bushings. Any brilliant ideas on how to free/remove these without destroying the bushings?

Thanks

PS I have tried some heat but not enough to damage the bushings. I've tried dry ice on the bolts in an attempt to shrink them and thermally shock the rust, and of course, I spray them with penetrating oil every so often when I am in the garage and it is on the lift.