In doing additional testing, this is what I've observed.
- The additional partition "Apple_Boot Boot OS X" is only there on my encrypted TM backup disks - it is not there on an unencrypted disk. On an unencrypted TM disk, the files to boot are there on the HFS partition, not in the EFI partition. I was able to boot from it into something similar to the Recovery Utilities menu.
- On one encrypted disk, as I mentioned in my post, the TM disk did show up on boot. It asks for the password early in the process. After going through a boot process that takes several minutes, I get the circle with the slash - so it has problems booting the disk. But looking at the way the disk is partitioned with an encrypted disk vs. an unencrypted disk and the files that are on this extra boot partition, it would seem that Apple's intention is (when working) it should be able to boot from an encrypted disk. The extra boot partition is not encrypted.
- On another encrypted TM disk, the extra boot partition was there but the files needed to boot weren't there.
- On yet another encrypted TM disk, the TM disk showed when booting but when asking for the password it said "[Update needed]". I stopped there - I might go forward to see what happens later. This was the disk which started as a Mountain Lion TM backup and it appeared the boot files there date to 2014, probably as part of the El Capitan update (I'm guessing).
So to me, it appears that theoretically you should be able to boot from a encrypted TM disk but the implementation is erratic. On the encrypted TM disk which has the extra boot partition but no boot files in that partition - it was the initial backup for what is going to be off-site backup so I'll just start from scratch there and redo the backup and see if boot files show up. If it can successfully boot, I'll try to do a file compare with the disk that doesn't boot.