This is the official response I got from HTC, after asking about usb host capabilities on the TP:
"The Touch Pro does not support USB host mode. There might be some third party applications that activates that functionality
, however HTC is not supporting it nor does HTC stand responsible for any use of third party applications."
This is really interesting; I was expecting more of an "USB host not supported. Period." Instead they hint that "third party applications" could solve the problem; this seems to confirm what I suspected: that the device is most likely only missing the proper driver. Again, since the class drivers (mass storage, HID, etc.) seem to be present (some drivers I installed myself, just to be sure) it's most likely that the low-level host controller driver is missing. Exactly which
driver that is (chipset wise) I can't tell, and with this kind of devices it usually isn't free/publicly availablea and, from what I read, notoriously difficult to reverse engineer...
I just hope I'm wrong and this can be fixed by building the correct cable...