How can I hardware protect a microSD from writing to
@Johnny above is 100% correct. The switch is not a switch at all, it functions in the same way as the tab on a floppy disk. With SD however there's only optional enforcement whereas with the floppy disk, the standard hardware enforced the write protect mechanism. 



A circuit in front of me wires the "SD Card Protected" line from the card cage (in this case, full size) to a GPIO port and the driver detects that. The picture above is to emphasis that the "card present" and "card protected" mechanisms are seperate from the MMCSD data transfer bus.

In your case the microSD card has no switch and no mechanism on the card slot to detect anything. Without looking at the BeagleBone schematic, I'd say there's no Protect line. But, that doesn't prevent you from


Creating your own card protect line using a spare GPIO line and using a jumper to select protected/not-protected. Then changing the SD device driver to use that GPIO line.
pretending a line does exist by altering the SD device driver
Not bothering and asking the kernel to mount the image read only


So I'm afraid the question reverts to a linux OS question.