Aye, you mean like this?
http://www.exilian.co.uk/forum/index.php?topic=2956.0
Cute, but no.
At that point, we should also consider a general mercenary center, buildable for all factions (or at least those on the list, a port respectively a center then), which would only allow access to a restricted numbers of units (the most popular mercenaries, say for instance Rhodian slingers, Cretan Archers and Thracian Swordsmen), but could be constructed everywhere.
Rather more wordy, but yes.
It's possible to do it all, actually. Merc Centers (recruits local area mercs), merc port (recruits "popular" mercs from all over - requires port) and even have mercs that can only be recruited in specific areas. (No need for two different merc centers, you can use a single "merc center" that just has the special options only in the specific cities.
Carthage would probably have somewhat expanded options for merc centers and merc ports, can use the same building as other factions though.
Depending on the balancing of Allied (AOR) vs Citizen (Factional) units, it might be handy to have fast access to mercs, granted the building would be quick to build, but cost a lot of cash...