I was looking into USB host support on the Touchpad and I guess I have some good news.
There is fully functional USB Host support, you don't even need to do any SW modification, everything is supported out of the box with one caveat, you need external power.
Support for keyboard, mice, harddrives and cameras is fully built in the default WebOS software.
I guess HP planned to sell their own USB attachments, iPad-style.
The only thing you need is USB OTG cable. Like this: New micro USB to USB Female Cable USB-OTG N900 | eBay
Basically just search for "USB OTG cable" on ebay, costs about $3. And USB Y-cable. The only two things you need are OTG microUSB cable and USB Y cable to provide external power.
I am still looking into possibility of self-powered usb host mode, with all the support HP built in for it it almost sounds like a given there must be a way to enable it one way or another. Unfortunately chipset documentation is almost nonexistent.