If everyone on the server is able to do this, or it is just a private server that is not open to the public, then it would be ok. Otherwise, I'd call it abuse.....not necesarrily malicious, but abuse none the less.
Let me see if i am getting the jist of this so far, If i use my admin ability to spawn in materials to build structures and what not for others players to explore (malicious or not) and us the same aforementioned ability to get the prizes for people for said events its still abuse and not enhancing the game play by adding more content and area's to explore?
Edited:
IF you build them then abandon them with no doors and no storage, then continue to build your own base somewhere else using only resources you've obtained yourself and NEVER return to the previous buildings then I guess it's fine.
But your setting yourself up for a distinct disadvantage. Someone could join the server, find one of these buildings and whack a door on it and there would be nothing wrong with that.
But IF you do return or plan to use these buildings for one thing or another, the sheer fact that you know exactly where they are and how they're built is your advantage. And therefore .. Your abuse.
It's better to just not do it.
I don't return to these places at all, Before when i originally posted this i didn't add doors. Now i do though but they are all unlocked so anyone can open them. Also like i said before i just build and only do the contest on this server. I also over watch the server for hacks and greifing.