I'm pretty sure a product publicly available before the patent was registered can't legally be considered to be in violation of the patent...

No, not legally, but they will sic their lawyers on you and bleed you dry anyway.

Being sued for something like this sounds like a perfectly good way to make some patent lawyer very famous and yourself very rich.

Maybe I'm just naive, though.

The problem is you will be up against a legal team funded by millions of dollars and they will get you stuck in red tape for years. I don't think a lawyer would take that on commission. And I don't know a lot about it, but I don't think counter-suits are very lucrative in patent law anyway.

More details on said proxy please.