Back up specific IMAP mailbox?
Here's my solution to my question:


First, I found that the contents of the IMAP mailbox I'm interested in are indeed stored on the Mac. Each email is an .emix file, and they are many many layers deep within ~/Library/Mail.
Second, I use ChronoSync to nightly create a bootable backup of the Mac's SSD boot drive. To handle this new email backup job, I created a Synch task in ChronoSync that nightly backs up this group of emails from within ~/Library/Mail to a new folder located on the drive which contains ChronoSync's bootable backup. Because the Mac runs SpamSieve for the IMAP email account, the Mac is always on and available to run this job in the middle of the night. 


Of course, I see now that having discovered where the emails were backed up already, I could as easily (save for the annoyance of navigation) find those .emix files on the Time Machine backup, or on ChronoSync's bootable backup. So it was a lot of worry for little or no gain.