Another Without-Unlock Backup Question: (Temporarily) Booting a Recovery?

There was a nice thorough discussion of backups for non-rooted (and maybe also locked) devices [here](https://android.stackexchange.com/questions/28296/how-to-fully-backup-non-rooted-devices) . Most of that covered Android 4.x+ because of the use of adb backup, which apparently doesn't exist in prior versions. A [second question](https://android.stackexchange.com/questions/33479/full-backup-of-non-rooted-devices-android-version-2-3-x-and-below) on backing up 2.3.x under the same conditions, found no answers. I'm am dealing with Honeycomb 3.2.6, which I have not yet unlocked, and I am considering [the last answer](https://android.stackexchange.com/a/159860/335152) at the first link. I am considering if there is a way to, possibly temporarily, boot a recovery to do the backup. I can see three possibilities: (1) That answer suggests connecting via USB with Debugging enabled and then:

abd reboot bootloader
fastboot boot twrp.img
fastboot reboot

As opposed to fastboot flash recovery twrp.img, that [should temporarily boot the twrp.img](https://android.stackexchange.com/a/181127/335152) without changing/wiping /data. From there it should be possible to do a nandroid backup to an SDCard. The author of that idea didn't seem to have tried it themselves. I wonder if anyone has, and if it's likely to work as described. (2) According to the [TWRP website for this device](https://twrp.me/motorola/motorolaxoom.html) , if I fastboot flash recovery twrp.img and then on reboot press the proper key combination to enter the recovery (in this case power+volume-down), then > Note many devices will replace your custom recovery automatically > during first boot. To prevent this, ... find the proper key combo to > enter recovery. After typing fastboot reboot, hold the key combo and > boot to TWRP. Once TWRP is booted, TWRP will patch the stock ROM to > prevent the stock ROM from replacing TWRP. If you don't follow this > step, you will have to repeat the install. Is this true for TWRP on all/most/many devices? (3) Another approach to temporarily booting a recovery, mentioned in one of the comments at that first link, is to boot one off of an SDCard. I found one example in [another device I have](https://twrp.me/bn/barnesnoblenooktablet.html) . I'm wondering under what conditions that might work and how to arrange it? If anyone has any insight, I would also appreciate them.