I personally like the idea of using concept art as a way to attract new readers. However, even though I am unable to support artists, I think it is important to have concept art exclusively for supporters. So if you had a mix of concept art for everyone, as well as some exclusive work for supporters, that would probably be ideal.
You could make a whole new series for the concept art, or just post it on your wall. It is kind of hard to attract readers to your comic though if your concept art is on your wall. I personally look for new series' before I go looking at the artists wall. That being said, I think posting some of your concept art on your wall for all readers is a good idea, and then maybe have a series based on concept art for your supporters -make them feel special
Just some ideas, though. In the end, any of those work -do what works best for you.