I have never tried it myself, but I think there would be some restrictions, even if Grub2 did support RAID-6 (does it?).
For Grub2 to work at all, it needs to load (at the very least) the
core.img which is usually 30K in size. However, there is only 4K available with a full disk mdadm setup (using
1.2 metadata). Thus there is no space to embed the
What Grub2 attempts to do in this case is map it directly to the drive where the
core.img is physically located in the filesystem. In theory this could be done even in a RAID-6 as the
core.img should be located somewhere in whole (unless your chunk size is smaller than 32K, anyway). That way Grub2 would be able to load it and boot as well, however, only as long as the disk does not fail, as there is no redundancy yet at this stage.
With RAID you usually want the box to keep working (and keep booting) even when a disk fails, so this is not a satisfying solution.
So if you want it to be reliable, at the very least you'd have to partition the disks to leave enough free space for embedding the
core.img. Personally I'm old-fashioned and create a small partition on each disk for a
/boot in RAID-1 mode (using
1.0 metadata so even non-raid-aware bootloaders can read from it). It lets the box boot at least into a minimalistic initramfs environment even if the RAID degrades for whatever reason.