May contain cynicism…
I'm not sure what could give you the idea that it would be a long-standing tradition to melt down everybody's jewellery to present a crown [at any temperature] to any passing claimant who's been promised one in exchange for his sister & keeps on nagging about it like a 12-year-old promised a weekend at DisneyLand.
I think it's fairly safe to say this particular punishment was thought up on the spot.
Think of it as the Dothraki equivalent of being shot in the head for saying "Are we there yet?" once too often. They are never portrayed as being particularly patient or tolerant.
I would consider the prior discussion as one of method rather than end result. The crown would amuse the Dothraki [and shock the audience] as being a fitting punishment, far more than just chopping his head off.
Similar to the warning, "May contain nuts". Doesn't bother most people, though some may find it detrimental;)