A third-party targeted decision might be the best solution for what you're trying to do, but then you can't select the target title, if case that's important. The war could be declared in script and it would allow you to select either the tributary or the target, while using the targeted decision on the other of the two.
It's definitely not possible to dynamically set anything in the war screen, and the war name doesn't take custom localisation either, so that possibility is right out.
I suspected that might be the case, but at least now I don't have to explicitly check - thanks!
Could you share what exactly you're trying to do? I've got a decent idea at this point, but a comprehensive overview tends to help a lot. That would make it easier to help figure out a decent way to implement it. You're right that making the AI be able to use a feature sensibly is often the hardest part of both designing and implementing it.
Overall, my goal is to facilitate (what seems to me like) a more-historical playstyle where you have a limited core territory that's directly administered (your demesne and vassals), surrounded by outlying territories which are very loosely controlled - you'll probably help them against existential threats, but you won't step in to solve every little issue they encounter. Basically, I think blobbing beyond ~50 duchies is both boring and unrealistic, and I want to facilitate some kind of "soft power" mechanic - eg. you might help to install your cousin on a faraway throne but then they're mostly on their own. (After all, the game is supposedly about dynastic success, not world conquest.)
So far, I've made a lot of progress by customising the tributary mechanics - it's not fully complete to my satisfaction, but it's working surprisingly well IMO, including for powerful AI suzerains. At the moment, this system helps me remain interested in a game for approx 100 years beyond my usual stopping-point-due-to-boredom, and I have started to realise some extensions which would make it even better. (Sure, I can force them for myself with the console, but I think it would be much better if they happened naturally without "cheating".)
For this particular development, I want to declare war on behalf of my tributaries. (As you've probably gathered!)
There are several reasons for this: (1) clean up bordergore; (2) have an occasional fun/challenging war; (3) expand my religion (for zealous characters); (4) prompt my tributaries to "do something", because they often seem to feel that they can't declare on anyone (if they declare on my other tributaries, they'll have to fight me, and I'm Very Scary; and their neighbours are also often quite large, so they don't want to fight them either). And then, of course, all of this applies to the AI as well - I want strong suzerain AI to create buffer states around themselves, and also to consider propping up their isolated co-religionists (eg. Crusader states).
Eventually, I want to be able to support my tributaries in all kinds of major wars - kingdom/duchy claims, holy wars, de jure county/duchy wars, maybe even invasions. However, initially I want to keep it simple and just get one type of war working properly (holy war), and get it working in such a way that it can act as a template that's easily expanded to other types of war.
After a lot of reflection, I'm pretty sure the only way to make this work for AI suzerains is to create CBs that suzerains can use to declare war directly. (I do not know exactly how AI decide when and where to go to war, and I'm not sure anyone does apart form maybe some of the devs, and I don't think it's practical to fully recreate all of that logic inside a targeted decision. So, the only real choice is to create a CB, because this means that the AI will use the same black-box logic to evaluate "should I declare war" as they use for every other CB.) And, as per the previous para, whatever method I end up with needs to be easily expanded to another 4-5 CBs.