No need for such backup. A sailboat is an excellent hybrid machine and tow boat insurance will fill any gaps. You may also have a portable outboard installation by installing the outboard on a dink and either towing or pushing the bigger boat. It can also be used to go ashore and bring in supplies instead of just hanging at the stern. A boat with a sugar scoop is perfectly adapted to be backed stern in into a floating dock; with an outboard in the stern you will give up that convenience. Lastly there is the issue of added drag should the outboard lower unit remain in the water. I would think that the outboard as a backup will not be needed frequently enough to warrant an installation.