You can't request an ally aid you as such... but allies can and will aid you sometimes. I guess you can simply assume you automatically request aid when you have a nearby allied army. Before I've attacked a city and a nearby ally has sent troops to assist. Also happened before with an allied fleet assistanting me.

But my current game as Parthia has taken diplomacy in total war games to a new level! I have a hugely rich empire in the east, with only one real land border, and I'm currently fighting a war CIA style against the romans. I'm technically not at war with anyone, but I'm bribing their armies, and giving the greeks huge sums of cash to fight the romans. Also trying to pay the Romans to attack Thrace... simply so I can fund Thrace to fight the Romans. I have bands of assassins and spies roaming roman lands terrorising the enemy. It's amazing what a level 10 assassin can do! I've pretty much destroyed the Brutii military without ever sending an army out.