It seems that the kernel code checking the back cover and disabling the mmc is here:

http://lxr.free-electrons.com/source...t/omap_hsmmc.c

Does someone more knowledgable than me can give a hint if there is a kernel boot parameter or some other way to fake the kernel into assuming the cover is closed?

On one of my n900s the sensor is broken (magnet still there, gconf hack does not work).