Brilliant theory. Madara, Kabuto, Orochimaru have always been crazy about 'enhancing' things with Hashirama cells. Now I'm sorta waiting to see if it'll backfire and kind of want to see if it has ever backfired in the past.
Now as I'm typing this, Shikaku used the rest of his time to confirm something with Kakashi about things that have happened so far in this battle and I immediately thought WOOD DRAGON. The way that it deformed as it absorbed the kyuubi's power until it was paralyzed.
When the Gedo Mazo swallowed the pot that the Gold and Silver brothers were in, they had Kurama's chakra in them from BEFORE Kurama's Yin chakra was sealed (I suppose Kurama could've been made whole again if he took back some of that Yin, but anyways...). If the Juubi were to absorb Kurama now, do you think he might go the way of the wood dragon? That fits with your theory. Shikaku's final message might've been to give Madara what he wants, the kyuubi.
The other way might be to let Madara and/or Obito absorb a Kurama-powered attack and get themselves some cancer. This could happen, because it does affect the absorber's body, like the Pain that turned into stone from absorbing Nature chakra from Naruto.