It is not (At least, it doesn't on mine) but that's not a Rockbox thing, it's an Apple thing. When you're connected in "usb mode" (where you can transfer files and it says "Do not disconnect") you're actually in Apple's firmware.
As I've never actually used Apple's firmware except to see what it looks like, I don't know if perhaps there's some sort of setting you can set there. But, if there is a setting, that's where it'll be.