Metro Plugin Referencing Unity API

In our company, we've got a C# library "Unity.Common.dll" that references the UnityEngine.dll and contains code that is used in all of our projects, such as utility code for creating game object children. This works quite well, until we hit Windows Store.

For Windows Store targets, it is recommended to create Windows Store Libraries and put them in the Plugins/Metro folder. However, in Windows Store Libraries, it is not possible to reference UnityEngine.dll due to the different library types.

Is there any known workaround, or will we be forced to stop developing with this approach and start copying Unity scripts back and forth?