I would contact a really good repair tech and see what he says -- like from one of the better known custom shops. I'm thinking of people like those at the Kanstul shop, Dan Oberloh, Dan Schultz, et al. It might be possible to repair that with minimal expense since you might be able to get away with just sending the piston to them. Also think about posting this to Tubenet (which those guys read) and seeing what people say.
I'd avoid going the JB Weld route except as a last resort for a couple of reasons. It's messy and might end up adding weight to the piston or introducing other problems. But the real problem is that once you add JB Weld into the equation, you've got a situation that no repairman will want to face. Getting that stuff off and then doing the correct repair with solder is very difficult and at least somewhat dangerous (toxic fumes and all).
I'm just thinking that you may be able to save this piston if you get it to the right person.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)