It isn't very clear if you mean putting that there with hammer or lua, so I'll assume you mean hammer. Actually editing the map will cause problems, making lua the preferred choice to accomplish this suggestion. The original suggestion, executed via timer, while not the best solution, would work just fine. Placing a brush entity with lua that blocks props would be the best solution.
Even easier would be to just use the same functions the server is using for other map changes like the innomotel boxes that spawn, I'd just freeze all the props in the barn personally at round start, much easier and less laggy. @Chai could I see that code or write my own?
+1 I love this map but it is really annoying when I get stuck in the teleporter especially when there is no staff online. Good idea.