As usual, being bored, I looked around at the US Patents Office and look what i found that was filed.
Let me quote:
A foldable unicycle attachable to a wheelchair to form a hand driven tricycle. The unicycle has upper and lower drive train frames. The means for attaching the unicycle to a wheelchair includes a positioning stand that holds the unicycle and its attachment means in a position adapted to receive and engage hitch means attached to the wheelchair frame. The positioning stand of the unicycle and the front castor wheels of the wheelchair are lifted off the ground during engagement of the unicycle attachment means to the hitch means of the wheelchair. The upper drive train frame includes means to allow it to be folded downwardly into a position adjacent the outer face of the lower drive train frame, and the unicycle attachment means includes means to allow it to be folded downwardly into a position adjacent the inner face of the lower drive train frame.