This is a really neat idea. Bitcoin already has transaction scripting built-in, in fact the regular transactions we know and love are just a special case of this scripting. The thing is, this scripting cannot take the amount of the transaction or things like the difficulty into account. It could, it's just not in the current scripting code. I think adding in some more scripting operations would be all that's needed to make this kind of thing possible. Not hard to code, but a big expansion of how bitcoin works, and that's kind of scary. Again, this could be a variant of bitcoin, like namecoin.