Multiple subprojects in Unity

I wish to divide my Unity3D project up into three subprojects; specifically, a client program, a server program, and a library that contains code both client and server will share, such as the communication protocol and such. Only the client needs Unity3D.

It seems like it would be nicest to have all these projects in the same Monodevelop solution, so they could all be edited and compiled together, trivially kept under the same VCS repository, etc. I created a new Unity project for the client, then opened up a script file in Monodevelop and created two more Monodevelop projects to the solution. It all saved, compiled, and seemed to work, but when I exited Unity and opened it again, the solution only had the one "Assembly-CSharp" project instead of all three.

It would appear that Unity3D has its own ideas about how the solution should be arranged, and re-creates it when the project is opened. Is there some way I can make it play nice with multiple subprojects, or otherwise arrange things such that I can easily work with several related libraries and programs, only one of which uses Unity3D?