Another possible microkernel for SqueakOS:

  http://www.chat.net/~jeske/VSTa/index.html

Here's a brief description from the site:

VSTa is quite different than your typical free UNIX. First, it does not even try to be exactly a UNIX; areas which have proven to be problematic for extensibility or efficiency have been changed. For instance, signals are strings, as are error numbers. Thus, VSTa is much more of a platform for experimentation with beyond UNIX ideas than simply another implementation of UNIX. 

The other major difference from "classic UNIX" is that VSTa is a microkernel. The microkernel provides messaging, processes, and virtual memory. All filesystems and device drivers run as user-level tasks. The kernel is 40K, and has remained at this size for quite a while, while the system as a whole has gained significant functionality...