I dont think it would deliver good maps. maps with many large outdoor areas already provide issues by being too minisentry friendly. And the indoor parts are often good to break up some sections. It at most could work for a detail contest.
But i still like the idea of a 'just lighting' contest. It shows a specific skill, and maybe we could even go further:
For that i have an idea that could be supporting: A fullbright detailing contest. Just show off your quality brushwork and detailing. No light allowed - in fact, you at most could enforce a night setting using some of the props for night maps (which are simply dark). Light props in this case can only be used as placeholders for lights.
The winning map (or a randomly picked one) then could be used for the lighting contest. This at least would allow normal light entities to be used. And if its good brushwork it could still give quite some good lighting options. The only exception to entities is that you are allowed to move lights.
A bit late since i commented on this thread already after this post... but:
Redetail a map that originaly was just halloween also is an option Skullcove for example is such map that originaly was only in halloween theme.
Its basicly just a theme conversion contest. And it can work in both ways.
Not for a playable map (or maybe you can do that), just for some sort of detail contest.