I'm pretty sure the author is aware of methods like Fanbox, but I can't find any indication as to whether they have one, so these are the only realistic options:
Art: Art is universal. I don't really know how to post on
Neetsha's art message board, but you can probably send those through the author's Twitter or email.
Comment: Feel free to express your encouragements over at the
JP comment section. Messages over Twitter or email might be a bit much.
Buying their games: Yeah, I don't really recommend this option because this really isn't how consumerism works. If their games are selling then consumerism dictates they will just make more games, and it's kind of weird to buy a product to support something else of a completely different genre. Buy their games if you like their games, but I don't recommend doing this if you only like Onnagu.
I still have hope even if the author has stated they used to do manga. If they could serialize Onnagu while making 2 games at once in the past then certainly we just have to guilt trip light that fire back in them.