I noticed that the U-Boot patch queue had patches to support discovering the maximum MPU frequency for AM335x evb and processor variants and setting the hardware clock to match, so I fetched these, applied them to the current U-Boot source tree and worked through compiling that on NetBSD. I now have an MLO/u-boot.img pair which will boot a BeagleBone Black into the kernel with the processor running at 1 GHz. It works on all the boards I have.
Works here, too. Feel free to fetch from here: http://www.ziaspace.com/~john/HOWTO.u-boot http://www.ziaspace.com/~john/MLO http://www.ziaspace.com/~john/u-boot.img John