How's this catch you? One person on DA said they'd pay to see that cross over. Sadly I can't think of a story idea outside of the TARDIS showing up in Rainbowland .
i'm pretty sure RBB would join against the Daleks in a second, they're all grey and oppressive...
maybe the the good Dr. follows an oddly rainbow colored battle cruiser and thinks rainbow might be in danger!
but she handled it, and doesn't really need rescued, so she lends a hand when he gets stuck instead.