There is a guy on this board that hacked a bike and inserted headset stems into the tubes. Find the post for details.
Other Separable bikes: Moulton Bike Friday iXi. I think there is a bike called puma that has some wierd cable system.
Gaitex may also be separable.
2000 Montague CX, I do not recommend it, but still ride it.
Strida 3, I recommend it for rides < 10mi wo steep hills.
2006 Rowbike 720 Sport, I recommend it as an exercise bike.
1996 Birdy, Recommend.
Wieleder CARiBIKE (folding), decent frame.