I made a secret room to my game and I want to put an easter-egg there and so I made a 3D voxel model of the crowbar from Half Life. I am allowed to put it in my game? It is just as decoration, no usability in game.

There's nothing in the rules that says you can't put easter eggs. So, put it in!

Easter eggs are great.

why not! 

Easter Eggs are always fun, and I'm sure none of the judges will mind!

Allowed and encouraged!

If your entire game was a easter egg i wouldn't bother!