I might be off here, but it seems that the answer is quite obvious: Look at reference paths in the project properties. In our setup I added our common repository folder, to the ref path GUI window, like so
That way I can copy my dlls (ready for publish) to this folder and every developer now gets the updated DLL every time it builds from this folder.
If the dll is found in the Solution, the builder should prioritize the local version over the published team version.