Anywhere you can shoot within the warday area is (to my knowledge) fair game.
Reds have myriad of way to counter sticky traps. Scorch shot/Detonator, bullet weapons, baiting the trap, ubers, etc. Not to mention a demo has to peak to trap highly contested areas, leaving them vulnerable to being sniped.
Inside armory definitely not. The blu doesn't have to detonate until they see rebel status, so in a way that feels like an extension of armory camping.
Warday is intended to be blus (with crits), versus a larger enemy team stocked with ammo. Quality vs Quantity. Sticky trapping armory heavily swings the balance of that, and just ruins the fun.