i posted this somewhere else, but here's how to get the brake arm off...
1. find a piece of pipe that fits over the coaster brake arm.
2. get a big crescent wrench.
3. sit down indian style on the ground.
4. sit the wheel in your lap, with the coaster brake arm at 5 o'clock, and put the pipe over it.
5. put the big crescent wrench on the nut at 8 o'clock.
6. grab the wrench with your left hand, the pipe with the right hand, and push.
7. the nut comes loose, and you bash both your fists together, and your left hand will scrape all the skin off the knuckles on your right.
but it works.