You could try using ipodpatcher (linked in the "manual installation" section of the manual). If this detects a macpod, it allows you to override and install the bootloader anyway.
But I also recall a bug in Rockbox Utility that will mis-detect an ipod as a macpod, and in those situations, ipodpatcher doesn't make the same mistake (which is hard to explain, as they share the same code). So using ipodpatcher may just work anyway.