"swap out the existing hub "
Sounds impossible to me. Some Internally geared hubs have the brake in a separate unit. I guess there might be some heroic technique to mate a derailleur rear wheel with an internally geared hubs coaster brake. Please post pictures if you pull it off.
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.