I just played with my h6315's registry (using PHM Registry Editor) and found this MinorClass key (in \HKEY_LOCAL_MACHINE\SOFTWARE\Widcomm\BtConfig\General) which lets you (only!) change the MinorDeviceClass of your iPAQ. Now I have to find out how to change the MajorClass and the ServiceClass to build something like BtClass. I tried to add several keys like MajorClass or ServiceClass but non did work.
I will keep on working on this.