It's an interesting theory, but consider Kairi. She lost her heart and wasn't put in a tube, but she didn't try to get her heart back, her body just remained comatose until her heart returned to her when Sora stabbed himself. Also, she kind of made a nobody. How could a nobody make another nobody?
EDIT: Plus, I agree with pretty much everyone else
Kairi was never a Nobody. Her Body and Soul was never sent to the realm of in-between. It remained in the realm of light.
And you can't attempt to get your heart back when you have a lifeless body....