Are homebrew developers allowed to submit assets of their games?

[puggsoy]

To my knowledge this is fine. As long as the assets aren't half-hearted edits (like recolours) and are actually original, you can submit them.
If the game is actually released to an extent and other people recognise it then you should be able to submit it as a game, otherwise it should probably go in the customs section.

You'll probably want to wait for someone else to provide input since I'm not 100% on this, but that's my two cents.