You can download the latest ipodpatcher (Mac OS X version) here:
http://www.davechapman.f2s.com/rockbox/ipodpatcher-1.2-test1.dmgWhen it detects a MacPod, it will now ask you if you want to convert it to FAT32. Note that this feature has only been tested on my ipod (a 60GB ipod Photo), but I would expect it to work on the Nano. The only ipods it won't work on are the 5.5g video ipods (the ones with 2048 byte sectors).
The instructions are similar to running ipodpatcher to install the Rockbox bootloader, but you now need to run it twice - once to convert to FAT32, and once to install the Rockbox bootloader.
Before you run it, you need to make sure that your ipod's disk is "umounted" - you can use the Disk Utility application for that - make sure you click "unmount" and not "eject". Also close itunes if it opens.
And please report back to let me know if it worked or not.