Unity reimports changed assets when it gets focus, so just switching over to Unity and then back to MonoDevelop will start the compile (you don't have to wait for it to finish before switching back). You could presumably write a MonoDevelop plugin to do this for you, but it seems like the sort of thing that would take more time than it would save. :-) Compiling after changes to C# code is generally very very fast.... I've got tens of thousands of lines of code in my Unity project, and the latency when editing a file and then switching back to Unity is under a second.