I am very happy to say that I have just proven you wrong in this regard.  It is possible to use your method to unbrick a DSTwo that was bricked due to a loss of power during the update process.

An hour ago, I was updating my DSTwo with firmware v1.19 on an old DS Lite when the battery died, effectively bricking the cart.  After a while of googling, I found your thread here and tried it out using an original (not copycat) R4.  While it worked and I was able to get the DSTwo to work again on the DS Lite, when I reset it, the cart was still bricked.

But I've found that it can be unbricked:

To unbrick with this method, copy a firmware update file (dstwoupdate.dat) to the root of your DSTwo.  Then follow the steps to get the DSTwo running again:



Once the DSTwo has booted, it will ask if you want to update the firmware. DO THIS!  Since the power cut out during the firmware update process, the cart became bricked because the firmware files were incomplete or corrupted.  Like trying to start a car with only half of the engine.  Updating the firmware restores the corrupted or lost files and allows the cart to boot again.

After the update was complete, I reset the DS Lite with the newly-updated DSTwo in it as normal (without using the additional flashcart and dstwo_load_NDSL.nds), and the DSTwo came back to life instantly!

Additional Info:
Before I updated the firmware, I had removed all files from the microSD except for the EOS files necessary to get it running (the _dstwo folder and ds2boot.dat).  After the update successfully completed, I copied a backup of all of the files in my previous DSTwo configuration (plugins, games, skins, etc.) back onto the microSD and it all came back to life, complete with my custom Fallout: BoS skin.

Thank you very much Ariel.  You've saved me a ton of pain and cash.