Many of the experienced mappers you're trying to reach out to would rather create brand new content that not only fit briefs that try to improve on TF2 play how they see it, but also have a chance of going official. This holds way more interest than working on someone elses (seen as broken) map that they're also aware is the target of a lot of disdain from the wider TF2 community, as already voiced by several members here (which i wish they'd have been more friendly about). This is a big thing for people who are vain enough to desire high traffic count on their maps (all year round). They'd feel like it was wasted effort.
A valid point, and one we struggled with last year. We had been planning on doing a similar contest last year for the design of a completely new Christmas-themed map (Valve's artpass contest scuttled those plans).
That's why we decided as opposed to asking for a whole new map (and all the work that entails) we would instead ask for a reinterpretation of our Halloween and Christmas warpath maps. That way, the only concern for the mappers is aesthetics, as opposed to design, etc.
At No Heroes, I think it's safe to say that we consider many of the mappers here true artists, and their maps (dozens of which we host) are pieces of art unto themselves. Last years halloween map selected by valve was stunning, and I can name dozens of maps we host that I've spent a ton of time on jsut looking at all the detail work.
Our only hope is that the compensation for the winners (which really are some kickass video cards - I just built a new rig with the same one) is worth their time. Perhaps when they see how much our community really gets into the holidays, it will drive home how much we'll really appreciate their work.