Basically you are gonna need a man or Chapter master of great influence to summon up a strength worthy of the Legions and right now the only ones I think that can get away with that are the High Lords of Terra.
Actually the High Lords of Terra are the last people who would be able to do that. Especially with the Wolves Templars and Angels. They have very little respect or patients for the High Lords.
As for the Unforgiven they do generally defer to the orders of the Dark Angels Inner Circle so it's highly likely that the Inner Circle could call together several Unforgiven chapters to act together as a Legion far more easily than other bunches. Especially if the fallen are involved.