With the release of U-Boot, USB Host mode (v1.1) is now an option on the Z2.
Previously it had been assumed that you needed to do a hardware mod inside the Zipit to get USB host mode working. I have since undone the hardware mod and have host mode working successfully. This means all you need is a U-Booted Zipit with a proper kernel and a USB to Zipit Dock Adapter.
The Zipit can only supply 3.3V when running on battery but the charger supplies 5V when plugged in. This means that if you want to use USB while the Zipit is not plugged in, you need a powered hub that has it's own battery or power supply. I managed to hack together a Zipit dock connector to a USB hub that has a power jack. It's not pretty, but it works. Foxx from #zipit and #zipit-dev created a nice mini USB adapter that fits inside the hirose dock connector.
GPSFan kindly did all the USB research and found the correct USB pinouts for the dock connector. The pinouts can also be found at the Zipit Wireless site (you must be a member to view the pinouts there). In the images below (courtesy GPSFan) I have labeled the USB pins and also outlined black tabs on the connector so you can orient it for the proper side.
Part numbers for the dock connector can be found here and here and here. USB and U-Boot "Shutdown" kernel patch available here.
Lastly, if you want to see it in action, check out my old video of USB Host mode and z2buntu here