Wouldn't it just be more intuitive to fix the wish spell?

I mean, we said arcane mark, yes, but that's just a brain storming idea. You could always have an improved version of the arcane mark that in order to overcome you need to beat the CL on the note. if you can fulfill the first premise of having a large institution that controls the money supplies, you can be pretty damn sure that the CL on those things are going to be fairly high. (I'd say as high as the highest level caster available in the guild)