There is a possibility to add USB host to the n900.
This might sound weird, but we'll need a seccond device... an access point, which also runs Linux, and has a USB (host) port...
I did something similar a few months ago (on another device)

I've submited a few ideas to the PUSH program, and If I get lucky enough to be selected and get a n900 (or a discount), I'll try and implement a virtual usb port for the n900.
This way, you can connect a usb device to a specific (but cheap) USB enabled access point, whose "host port" is then "forwarded" to the n900, via WIFI.

keeping my fingers crossed..