Ok, I think I know what is wrong. Because the class hierarchy is split between jar files, the classloader being used to sequentially open each jar file and examine contents for CLass.isAssignableFrom(Class) is to seeing that the subclass of BaseDriver is also a Driver. I will add a bug report on this.
In the meantime, there is a work around for the problem. Unjar all three jar files and then create a single jar file with the content of all three. When you open it in the Driver editor you will now see the SQLServerDriver in the list of available drivers. I have verified that this works.