xpcshell tests for electrolysis

Last week there was some discussion about testing of electrolysis.  One 
of the ideas was to use xpcshell to test some things across processes, e.g.,

   1. set a preference in chrome using nsIPrefMan (or whatever)
   2. send an ipcshell command to the child process that reads the 
preference and returns its value to the chrome process
   3. in the chrome process, assert that the value is correct

This kind of test is now possible, thanks to BenT's work in xpcshell.

Now I'm looking for ideas for other kinds of electrolysis tests that 
would be interesting to perform in this context.  Any suggestions?

Jonathan