This type of tutorial is invaluable. The man page for 'mdadm' is over 1200 lines long and it can be easy for the uninitiated to get lost. My only question when working through the tutorial was is it necessary to --fail all of the remaining partitions on a disk in order to remove them from the array (in preparation to replace the disk)?  The answer is 'yes', easily found in the man page once I knew the option existed.

One of the follow-up comments included a link to a post from the Linux-PowerEdge mailing list entitled 'Sofware Raid and Grub HOW-TO' (yes, 'software' is misspelled in the post's title).  Althouth this paper is dated 2003 and the author refers to 'raidtools' instead of 'mdadm', there are two very useful sections. The most useful is on using grub to install the master boot record to the second drive in the array. The other useful section is on saving the partition table, and using this to build a new drive. (In my own notes this I add saving the drive's serial number so I have a unambiguous confirmation of what device maps to what physical drive.)

Merging these tips to Falco's instructions gave me a system bootable from either drive, and easily rebuilt when I replaced a 'failed' drive with a brand-new unpartitioned hard drive.

Thanks to Falko and the other helpful posters.