You can modify grub to prompt you for a password to mount the volume, but the volume would have to part of your file system i/e have some files saved on it already and be included as one big single disk. Otherwise Grub would need the disk UUIDs and initrd from your OS kernel would need the volume UUID, which you could do with an SELinux loopback to grub kind of like an ISO setup. Jerry may have a better idea. Right now you're on the media channel, that is your RAID device is being treated like removable storage which has some advantages (protects your OS from disk failure problems) and some disadvantages (does not mount at boot with the OS). I don't know what you're saving, what data you intend to store on the volume but you might want to look into Debian open media vault instead. Really pretty easy to set up and use.
TC