Personally, temporary hit points seem like the way to go. Maybe just that at the earliest levels, but later on, it adds a buff of choice, with more choices for buffs as the character gains levels. Or even make it something that can be improved through expendeture of a resource--spend one Inspiration point/die/unit/whatever, it's temporary hit points, but spend two and add a buff. The only problem I could see is that buffs, granting saving throws, and extra actions feel like the territory of other Warlord powers. And while not being able to use a power that heals a downed ally is rough, something like Manbearcat's Don't Die On Me to possibly prevent death would be a good substitute, and then the Warlord could have additional things to help characters that are unconcsious. Possibly something called "Shield the Fallen" or some such that lets an adjacent ally take the attack and/or damage that an adjacent ally at 0HP would have taken to save that person an automatic failed death saving throw, but it can't be used to protect from an attack that included both characters.