Have you guys considered creating a Steam Workshop page for custom brushes? The weird and unique brushes are my favorite part of Black Ink, and I think that having them be easily accessible on the steam workshop would encourage more people to share!
I'm not sure how complicated the implementation would be, but this guide isn't too long : https://partner.steamgames.com/doc/feat ... ementation
I've seen a lot of games get really big publicity when they open their steam workshop pages, so I look forward to a big spike in users and user-made brushes!