I've been in quite the same situation exactly, but with the Byzantines instead of the Poles and the Turks in place of Milan... I myself accepted to assist in the attack and took place with one of my stacks in one of the Byzantine battle against the Turks... My overall reputation took no hit...(maybe the fact it was a muslim faction helped) but I gained no thing from the Byzantines either, my standing stayed on Very Good...

On your situation, the only consequence to your reputation might be with the Pope who might not take lightly your joint attack on Milan (catholic)... But if you grease his palm sufficientl, you should be OK...