I don't suppose there's anyway for an "event" of sorts to happen whenever an ally goes to war that says, "Do you want to join X in a war with Y?"
Also, when you get into a war, the AI would get sent a message saying "do you want to join X in a war with Y?"
Sort of like what happens in Europa Universalis.. only I wish there was another option when the AI refused to join that says, "do you want to break alliance with them?" instead of breaking the alliance automatically. I would understand if Spain didn't want to help me combat Denmark, when I'm HRE.. since Spain is so far away (or maybe they're weak) but perhaps I would rather keep them as an ally for my plans against Genoa at a later date (which hopefully Spain would help me with at that point in time.)
Make sense? Hope so. I basically am just trying to find a way that allies can be useful, other then a means of being 'fairly positive' you won't get attacked from a certain front.
I do realize that if I wage war with Denmark and am allied with Spain, I'm pretty sure they would hate Spain, also.. but I'm looking for things to go a bit further then that, and perhaps even allow for a bit more "role-play"