Strengthen or preserve a soap carving?
If you weren't planning on painting your carvings, how about varnish?

A transparent and colourless lacquer should be almost invisible after it's dried on the finished product, but will adhere to the soap and preserve your carvings as well as keeping the moisture inside the bar and preventing it from cracking.

From J. C. Rich, The Materials and Methods of Sculpture, p. 357 (emphasis mine):


  Soap is an inexpensive, readily available and easily worked material that is steadily gaining in popularity as a wholesome and interesting avocational carving medium. (See Plate 62.) The major disadvantage of the substance is its marked lack of durability. A work in soap is subject to a degree of physical shrinkage as it dries out and this may result in the development of fine surface cracks, which may in time fracture through the entire piece. As a soap mass dries out there is also a resultant change in appearance, from the soft and attractive surface the block has while it is fresh and possesses an abundance of moisture, to an opaque and dull surface after the bulk of this moisture has naturally evaporated from the mass on exposure to the atmosphere. However, a finished soap carving can be treated with one or two thin coatings of colorless and transparent lacquer, which will serve to prevent the marked evaporation of moisture from the mass. Much of the attractive appearance of a feshly carved block of soap can thereby be preserved, and the permanence of the work substantially increased.


And this tutorial from the LDS organisation website tells you what to do after you've finished the carving process (again, emphasis mine):


  Set your carving aside to dry for several days and then polish it with soft tissue paper, using your fingertips and palm of your hand to bring out highlights. If you wish, you can preserve your carving with a coat of transparent lacquer or tempera paint.


(Oh, and just for fun: Has a prisoner ever escaped by carving a bar of soap to look like a gun, painting it with shoe polish, and then tricking a guard into allowing them to leave the jail; or is this only an urban legend?)