Related to this is the following idea.
Mercenary contracts played an important part in the history some countries; especially for the Swiss federation it was a major source of income. These contracts were basically non-aggression treaties between two countries. In addition to that one country received the right to recruit mercenaries in the other country against the payment of a yearly sum. The other country was allowed to call back the mercenaries in times of need. It would be nice if they were implemented as a diplomatic option.
The effects would be some kind of royal marriage. The recruiting country would raise its manpower-threshold against payment of some money (less than the excessive manpower penalty, or else it would make no sense). The other country would get the money; I am not sure about lowering its manpower-pool though, since having mercenaries abroad which could be called back resulted in an increase of military strength.