Did you ever found out how to do it? I find warjort's answer a little lacking in details.
I think the best answer would be a link to source code or a slice of code that gives an example of the following:
1. A pack.mcmeta file where a single file or directory is defined as the custom sprite to be added.
2. The path relative to the resources folder of the actual custom sprite file.
3. Code in a working mod where the sprite is loaded and used to draw something
I hope this is not too much to ask I really have no clue what to do with the information that warjort gave there are just so many things that are not explained.
