Sorry, the Obj-C stubs that MonoDevelop exports to dummy Xcode projects to make the Xcode designer aware of user-defined types do not currently include types from libraries.
You should still be able to use types from libraries but you'll have to manually type their Register'd names into the "custom class" field in the Xcode designer. Unfortunately connecting outlets and attributes would not be possible, but the designer would not remove existing connections.
A way to work around this limitation might be to create the xib in project 1, and set any custom types on controls, and connect outlets and actions. Next, remove the xib from that project, and copy it into projects 2 and 3. In those projects you should be able to modify properties of the controls on the xibs without breaking the outlet/action connections.