as per Intel datasheets, the GM965 chipset (afaik that's what's in the Mk2) only supports 4GB.
I'm curios if it works (it wouldn't be the first time the specs are not updated for older stuff), but I wouldn't hold my breath.
that's a good MB design in toughbooks - having daughterboards for external ports like USB, that may be subject to physical/electrical damage, for an easy/cheap replacement.
you wouldn't want to replace the whole MB for that.