I can't think of a single cutoff point for evaluating "participation" that won't result in the occasional garbage interpretation of "you didn't help us!".
1. England declares on Tyrone and calls you in, it stack wipes Tyrone and sieges. You didn't help?
2. You declare war on a new world nation and carpet it, but it takes a long time. You call in Spain, but they never sent troops. Penalize the AI here?
3. Austria as HRE calls you into war against France (you're Serbia). You send 2 guys into France and they siege for a moment before getting stack wiped. Legit help?
4. Muscovy calls you in on Uzbek (you're Byzantium). The war lasts a few years but it completely one-sided in their favor, so you can't actually fight units even if you try to get stuff over there. 2 man siege good enough?
5. You're losing a war badly and get a CTA. You have almost no units. Dishonored alliance for non-participation in a 2nd war you honored?
6. You are Aztec. Spain declares on you (or you on Spain), bringing in derpscrub Portugal for the ride (I love how people blather about westernization being ahistorical, but then don't have anything to say about a combined alliance against the Aztec shipping 40k-60k + calling crusade for bonuses by 1530. The game is what it is ffs). Portugal, having just lost its navy to a spat with France v. England + getting its fleet caught, has no active navy but of course still joins the war because it's otherwise in good condition. As a result, Portugal "doesn't participate" as the player Aztec blasts Spain's fleet out of the water and starts taking colonies. Iberian alliance shattered when this is "engineered" to happen a few times?
Pick one or even several criteria that covers all of the above and will consistently penalize non-participants appropriately. It's not going to happen. Short of deliberately making the AI cheat and forgive other AI here (and not penalize it for not helping humans), creating an arbitrary, hard to understand rules framework, this kind of mechanic is going to wind up being a non-starter. You are virtually guaranteed to have situations where the penalty will be applied inappropriately and ruthlessly abused, unless the criteria for participation is so lax that the mechanic is functionally pointless and a CTA just results in a player occasionally attempting to loot a few provinces.
It's a non-functional concept under the current war leader and negotiation rules IMO. I challenge anybody capable of demonstrating a model whereby I'm wrong about this. Maybe I'm missing something. If I am, show me. If not, it's not justifiable to introduce a mechanic whereby players are encouraged to "pretend" they're helping "just enough"...it's not going to resemble anything historical and it's instantly going to sit in the "game around this but not actually participate" category, because if that's impossible you're going to get complete garbage for "non participation".
Per Targor's suggestion, I can force the AI (who can't fight battles) to break an alliance between historical friends so many times that it alters the diplomatic landscape in Europe, and I can do it as a 4 province minor in Asia in the late 1400's. The OP is no suggestion at all.
That's relatively simple. It simply evaluates your ability to participate and how badly the AI really needs assistance.
Does war leader require help?
-Measure enemy alliances units, force limit, manpower, and province wealth
-Measure war leader units, force limit, manpower, and province wealth
Do you have the ability to help?
-Is it possible to get troops to the enemies, IE via Navy, access through another country, or direct access to the country
-How is the players (or whatever countries) manpower, force limit, and wealth vs the enemy countries
1. England doesn't need assistance, it's force limit and provincial wealth are significantly higher than Tyrone's and England obviously doesn't need help.
2. Again, aid never needed, and Spain may not have access to that new world, limiting it's ability to participate
3. Your ability to help is limited, you, a weaker power, stand no chance against the mighty power of France
4. Muscovy doesn't require help. Though it does take awhile getting access is a problem, and they are more powerful than you. But as you said it is completely one sided
5. Your ability to help is limited, your provinces are beseiged and in all likelyhood your manpower is in the pooper. Even if they do call you in you lack the ability to help.
6. Portugal doesn't have any way to access the country
However this is assuming that this is all just a relations hit so that it can be scaled, sometimes you'll go above and beyond the call of duty, sometimes you won't do a damn thing while your ally desperately needs you.
But as Josh127 it just seems way too big a pain in the butt to implement for a feature that really isn't that big of a deal. Sometimes realism simply needs to take a backseat to game play