If you can drain and clean it you could bring it to a plastic bumper rebuilder like we use and they could "weld" it very easily.
As far as the company you bought it from I would think about making up a sign explaining this problem to put in your window at rod runs and shows. A very nice not kicking their butt, just explaining as you did here. I would then email them a picture of the sign in your window and explain how you will be going to X amounts of events a year in this car and want to know if they would re-think replacing this gas tank.
Now mind you, this has to be done very tactful and without any "opinions" expressed, only facts. When you bought it, for how much, the name of the company and the results, with a question "is this fair?" kinda thing, not "They suck don't buy from them". Throw in a few "this is a serious safety issue, we are talking about leaking FUEL".
You have expressed it very well here, you aren't beating up on them screaming, you are explaining it very well and I can't imagine any kind of legal issue. Unfortunately we have to think about that these days. But that being said, I think they would be damn stupid not to replace that tank.