If that were the case he 'probably' could have done it long ago. 7 tailed beasts have much more chakra then nagato and he recently obtained the rinnegan. Tobi's main goal is to become complete/eye of moon plan, how could madara contribute to these plans?

Madara must be allied with tobi, but just using extra chakra seems too simple to get the technique to work. What was the point of all those zetsu's? Unless Tobi is still unable to use the rinnegan to its full capabilities then this seems unlikely, but not impossible.