You don't want to change mount order for that (mounting is unrelated to usb).
If you uncomment the #define HIDE_FIRST_DRIVE in usb_storage.c and recompile, you'll have a build that behaves the way you want for USB. Of course this disables USB access to the internal flash entirely, so that's probably not what you want.
You could install a regular build, and copy the rockbox.mi4 file from the special build somewhere on the device, and "play" that to boot into the modified build. You'll then have to reboot to get back to the regular build again of course when you're done with the USB thing.
Or you could add an option for this. That shouldn't be too hard. Maybe I'll find some time to add that