One of our random google adds at the bottom had a link to sharpie art.
http://www.sharpie.com/enUS/Pages/ArtworkDetail.aspx?eid=873376 so I clicked on it to see what they had. It was pretty interesting. I like what they are doing to help market what you can do with sharpie. Could be a good thing for you screen printers to do for your customers. Sort of to have a
wall of fame on your website.
Encourage your customers to take a photo with the customer wearing the printed tee or with the art they used for the tee shirt.
Side note, When I was at Disney, we did a departmental team building event. The department heads bought everyone white high top canvas tennis shoes. They got your size and took orders in advance. The images attached are not what we did. Just exampled from the net to give you an idea.
The thing was to decorate them yourself during the designated event day.
I knew I wasn't keeping mine so I didn't decorate it. I donated mine to a friend of my son who needed some shoes. So I got mine in his size. Any how, to my personal taste, I liked many of the ones who did them in nothing but pencil. Pencil on canvas works really well. Some people painted them with Disney characters and others did flowers and bedazzled them to death.
All in all, it was a pretty cool event. It sounded pretty ridiculous at first but it turned out to be a fun experience for teams. So, just another thought. You might want to do something ridiculous and paint/screen print on your shoes.