Love this idea! people have a lot of success with mail on Patreon (this post might interest you) and I believe we’ll have a blog soon that might have some more ideas about this.
I like the idea of art on front with Patreon plug on the back. They’ll want to display the art in their home and be reminded of your Patreon whenever they look at it. I like @ginnydi’s idea of adding an extra little something to encourage that sign up - a special summer offer perhaps or “as a thank you for being a previous customer of mine, if you join my Patreon page you’ll get X”.