I've got a couple of file-servers that only have a RAID-1 partition for grub. The entire OS and main storage is on the RAID5/6 array. Only the 2-5MB raw kernel and initrd images need to be off the RAID-1. Which would more-or-less stick you in this situation with trying to re-direct the backup file for a RAID5->6 conversion.
If all else fails though, a Gentoo Minimal Install CD generally has the latest MDADM tools installed, then the conversion can occur 'offline' instead via the USB storage to hold the backup file.