Actually, as shown Madara needed to wait until the Juubi was *reduced* to the Gedo Mazo via multiple bijuu extraction for summoning to occur. Gedo Mazo is literally no more while Obito is the Jinchuuriki of the actual Juubi.
If Madara couldn't even summon Kurama from Naruto, what makes you think he can summon the Mazo, as the Juubi, from Obito's body?
You're right, we don't know the limits of Limbo Hengoku which is why, more often than not, that one technique won't ascend him to Juubito's level. That's all I was saying on that matter. I think the possibility exists for Madara to be above Juubito if not one but both of his original Rin'negan eyes return to his sockets. As Madara stated, dojutsu users need both of their eyes to unleash their true power — so while we do not know the limits of something like Limbo Hengoku, we know Madara will be limited to the extent of having one Rin'negan. I don't believe Madara will be above Juubito until he has all of his powers complete and in tact.