Voltage is controlled inside the bmminer code, as is fan speed. I don't see any active way to modify them short of a recompile of bmminer, which isn't too hard.

However, you can skip all this downgrading shenanigans if you want, since the rootfs is ubi and mounted rw. --fixed-freq is the required command line for bmminer, which is called by single-board-test (statically in the binary). If you create a shell script (/usr/bin/bmminer) that calls the original bmminer executable, it will be run and you can intercept and change the parameters.