I did some googling, because I'm curious. Tried micro tagging, cel phone charm lanyards and custom zipper pulls sliders--which are awesome but seems like it's more expensive ($250 for batch). It all depends on how polished you want your swag to be. Do you want to make your label or order from china? Can you order a custom stamp and punch/stamp your own? Size will be an issue. Maybe you'll find a micro, waterproof tag that can be printed on...
http://www.myassettag.com/BC/Laser-Barcode-Labels.aspx
And maybe you can approach a print shop who might be able to print something super sharp, with numbers.
Your tag is not bad by the way, and I don't think a hand written number makes it look bad. A new material would help with durability, and if you somehow, pre-cut notches in the middle where it folds, or can find butterfly shaped labels you could have a slightly bigger print space and still fit it through a zipper.
It's fun to think about, isn't it?