The N900 does NOT have USB OTG, the detect pin is not even connected.
We have a cool USB driver hack by JoergRW that gives us USB functionality but it is not true USB OTG, though in some ways it is more useful as you can toggle the mode you want rather than having to rely on it automatically switching.
That said you need to be booted into Maemo and have the software installed(h-e-n and/or USB mode).
There is no USB OTG or host mode functionality in U-boot or BackupMenu.
There are options to get U-boot to boot from the SD card though.