On the N810, in the Settings dialog of Maemo Mapper, enable the "Bluetooth" option and leave the corresponding text field blank.

According to the published SDK API, this is supposed to cause Maemo to search for any GPS-enabled bluetooth devices in the area, but for some crazy reason, on the N810, this configuration causes Maemo to use the internal GPS receiver. Sorry, I know it's confusing, and I have no explanation for it.