Never mind what the iriver site says. You can update your firmware to use the international versions, but just bear in mind that doing this will permanently render the player useless for DRMed music. You also need to use an international firmware in order to be able to install the Rockbox bootloader; the U.S. firmware versions won't work.
I never really used the USB-OTG function on the iriver H300 series (my girlfriend has one but she doesn't bother with it either), but I can say that USB-OTG support isn't in Rockbox yet, but it may be soon. How soon, I can't say.