Quite possible to customize! Would not be an additional cost and would be easy to do yourself if you have photoshop or something (I would help out with it but I've got a ton of other projects to complete). I can send you a PNG master file (I can edit it for you within reason, as long as it doesn't take too much time). Then you can choose a tshirt printing site of your choice and have it delivered to you. We'll be using http://www.spreadshirt.com.Looks interesting. If there was a way to personalize it without it costing too much more, that would be grand : P.
Also, see you at Minecon. (Tickets did sell out too fast, but not too fast for me!)