The application I'm using this for is simulating a small system (embedded CPU) which just happens to be in C.  The nodes are connected together with radios (think Crossbow or Dust Systems), and I'd like to simulate a network of them, while letting the Ruby code record the radio traffic to a database for later analysis.  When they make a call to their "Send a radio message" routine, I just catch that in the Ruby and then deliver it to whatever nodes are "close enough" to hear it.  I think I can make it work using the FFI and call-backs.  I've found that I can use the library approach if I make a discrete copy of the library for each node; each will get mapped into the space independently.  The global variables don't clash because all the linking is done at runtime; I'm not sure exactly how it works, but it appears that the global vars are not added to the processes global vars, at least as far as I can tell.