I know that the student would be studying a related field; that was not the point. I as a hypothetical viewer would not care what the grad student was studying, exactly, I would care that he was only a 20-year old graduate student still studying at a university (that I would assume to be populated with liberal professors).

"Winners don't win by playing dumb."

And that is why I don't get this proposal. It is assumed that this college student would absolutely destroy the creationist debater and persuade the open-minded and objective audience through sheer, well, persuasiveness. But the audience, unless already completely in favor of evolution, is at least sympathetic to the creationist and interested in their views. This proposal would signal that this experienced debater and high-status leader of a movement is no more than a wet-behind-the-ears, unexperienced student. Doubting listeners would dismiss this fact out of hand and a priori; they will think it condescending to send someone like that to debate someone like this, which it is, to the creationist but especially to the audience. They will then attach less weight to any arguments, however persuasive, the student would make.

Biasing your audience against you before the debate has even started is not a viable tactic.