Wow, this was a quick one! Other than a simple check, there are no major blocks to changing the default sounds.

As of right now, a custom sound has to replace an existing sound, and take its name. In my example I replaced s01_ power_on.mp3 with my own mono mp3. I hade to mount the userdata partition in the tmp directory, and replace the desired mp3 in the /tmp/userdata/music directory. 

It also seemed that I needed to replace the same file in /mnt/udisk/music directory, or else I had issues where it would grab the original, or not play at all.

Here is a video example: