It depends how the internal microdrive is interfaced. The XScale CPU/chipsets can drive two PCMCIA/CF slots. In all other Zauruses the 2nd slot is unused, but maybe they used this for the internal slot.
If this is the case, then the internal connector would be like another CF slot and you could put whatever you wanted in it, as long as it could physically fit in the space available.
But if you can cut a WiFi card down to fit in the same space as a microdrive, why not just put it in the external CF slot??? Also if it is possible to do this, then why haven't any companies done it, like Socket did with their Bluetooth CF card?