Sounds really interesting, especially if it required good planning for the Civ to develop and stockpile the necessary equipment to make an effective temporary army. I take it you mean that the population/productivity of the City would be reduced whilst conscription is in effect? But would the A.I.be able to use such a system? Another cool project could be to prepare for a siege, where the city would stockpile food resources and construct defenses, whilst sacrificing other aspects. Especially if a cities access to all it's tiles could potentially be cut off, and not only by direct blocking/plundering of tiles.