Actually, this idea was very similar to my one of my maps. In q3f, I created a map called "The Mountaind Dew Wars". Themed, Red vs Green.
Anyways, there was an air tunnel chute that led from the lower respawn of each team all the way to the other flag room. The entrance to this tunnel from the lower respawn was sealed off until a button was pressed in the base. Once that button was pressed, the seal was opened and a well organized offense would all literally fly into the enemy teams flagroom. Of course this tunnel only stayed open for a short amount of time.
The exit of this tunnel shot you across the flag pole stand and towards the exit of the flagroom leading to the ramproom. A little bunny hopping, and you could be in and out in a dash.
Also, there was another button that opened up an entrance from the ceiling of the flagroom, again another place where a well coordinated offense could storm the enemy flagroom. Again, only open for a short amount of time.
(the amounts of time each button opened the above entrances, tunnel and roof entrance, was determined through lots of play testing)
The idea of the spies you had was similarly implemented in my map as being, no one was allowed in enemy respawns, except for enemy spies. If a soldier went in an enemy respawn, they were immediately teleported to the "Mountaind Dew Vat", where green burned red and red burned green. Each teams dew vat was also the cap point for each team.
If I were to make a map, I'd remake this map. But I'm too lazy! If anyone else would like to take the theme....
