I think those ideas sound good - but I think everyone competing should be made to wear a pumpkin head to make it even more "scarey" >:D
I presume this is best held on the 1.7.10 server? - if so - you could use my prison arena (warp to "theprison") where you can turn the lights off in the arena (warp to "theprisoncontrols" to use the switch).
I give permission to temporarily modify my prison arena to place spawners, provided that no existing blocks are broken in the process.
I best get harvesting some pumpkins