[OnyxCody]

I've recently been adding proper MPAA ratings to as many of my "Rated NR" or <blank> films and series (tv shows) as I can find,

Do you have your scraper correctly set up?

yet after finally learning how to find and decompile the proper texture file for the Aeon Nox: SiLVO skin, and looking at the "media\flags\colored\mpaa" directory, there currently doesn't appear to be images for the MPA Canada ratings.

Yep, that is not surprising. Most skins only have content rating icons for a few countries at most. It would be a bit hard to add ratings for 200+ countries. So you are heading down the right track by sourcing and preparing your icons. Make sure they are transparent png's as I can't tell from your screenshot.

how do I get the Aeon Nox: SiLVO skin to recognize these new images;

You need to edit your skin files. The mpaa bits are held in variables.xml here... https://github.com/mikesilvo164/Aeon-Nox....xml#L1039
They are in a few locations in that file, so do a search for MPAA to find each of their locations.
Slot in your Canada ones, either by adding new lines, or editing existing countries that you won't use.

if I add these 6 additional images to the "media\flags\colored\mpaa" directory and then re-compile the whole Aeon Nox: SiLVO "skin.aeon.nox.silvo\media"

You can use the TextureTool... https://kodi.wiki/view/TextureTool
But if you add the unpacked files back where the xbt file was, keeping all the proper folders in place from when you extracted it, it should still work. But it will prevent future updates. So up to you which way to go.

@Karellen Thanks for the directions.  I'll get started on that first thing in the morning. 

I actually found the TextureTool because it was mentioned in one of the forum comments after spending some time looking into where to find the proper directory/file which contained the mpaa files. So that is how I was able to discover that the MPA Canada images didn't exist yet in the default skin setup. 

I would prefer the default Aeon Nox: SiLVO skin to be updated to include these additional images I created. However, until that happens, if it ever happens, I don't mind making this update every time the default Aeon Nox: SiLVO skin gets updated. I'll just have to turn off "auto updates" so that I'll be ready to make the manual changes right when I implement the skin update.

Thank you very much the advice.  I'll report back hopefully tomorrow morning with a screenshot showing it working with the new MPA Canada mpaa colored images.
I just love the customization capabilities of Kodi since I started using it on my first small NAS box 5 to 8 years ago. Especially since I've been getting more and more particular with my custom setup this past year and I've only scratched the surface of what I can do.  Keep up the great work Kodi Team!