Just wondered if the whole publicity thing might be a factor.
By that, do you mean if the manufacturer chooses not to give you a new frame (which they are well with in their rights not to do) that you will bad mouth them on the internet? If so, then I doubt you’ll be successful. Nobody falls for that any more, and I think you’d be shot down pretty quickly on here too, as you’ve already stated the frame is 2nd hand.
Or, do you mean that you’ll rave on the Internet about how great they are, because they gave you a new frame? If so, I’m not sure that will work either, as they’ll have every Tom, Dick and Harry on the phone asking for free replacements of their 2nd hand frames too. Not really a good outcome for the manufacturer either.
You basically don’t have a leg to stand on, so if you go in guns a’blazin, expect to get knocked back down pretty quickly.
The best thing you can do is be honest, and approach them in a reasonable manner. Most people who work in the bike industry are riders too, so will understand your position. I’m sure if they’re able to help, they will. If they can’t though, don’t be too surprised.
As matt_outandabiut said, 2nd hand is cheaper for a reason.