Hahaha nice to hear that you are enthausiastic for this contest. The skin would be a nice step upwards to making this a more proffessional BattleBot contest. It would be be a nice feature in multiverse for admins to select the allowed blocks and their limits. This would probably be up to the devs or someone who likes to make a mod for it. I've been looking into creating the mod myself but im not (yet) that familiar with scripting in Unity.
as for the having allowed blocks or limits, this probably won't be possible because 1. the devs already have block limits, nobounds 'gets around this', and mods probably won't work serverside, everyone playing would have to have the mod themself