Hmm ... the reason that iron would be the cutoff point is 'cuz elements below irongain energy when they fuse, whereas elements above iron lose energy when they fuse (I think that's right). So basically you can't just take two elements together, bang them together at high speeds and get an element with atomic number higher than lead. It takes a certain amount of energy in addition to the two elements. The reason it only occurs in supernovas is that stars convert hydrogen to helium (via nuclear fusion) ... that's the majority of their life cycle (and the reason why those are the two most common elements in our Solar System). As the star gets older it moves to ther elements ... at some point it begins to fuse iron. (Mind you it only progresses to this stage if it has some critical mass equal to 2-3 times the size of our sun). It can't progress past lead so it "cools" down and the fusion process stops. This causes the star to contract (since the gravitational pressure is no longer countered by the heat). The resulting contraction will cause a huge amount of heat and since the iron can't fuse anymore it explodes. If the star is less than 5 solar masses or so the neutrinos can prevent the core from collapsing (the star will still supernova though) and it becomes a neutrino star. If it's more than this however, it becomes a black hole. Interesting stuff, eh?
But to negate your theory I'm pretty sure it's iron that is the cutoff point in this process so I don't see why only supernovas can produce elements w/ atomic number greater than lead.