Ok, here is the answer. Silverlight class libraries are compatible with all .NET platforms (since they are versionless). Just do it, compile the class library for the assembly, and then add the link (instead of adding to the project).
The Silverlight class library provides additional references (for assemblies that target the Silverlight platform) backstage, so Silverlight also needs these links to work properly.
Create an instance of the class again (there is no easy way to do this)
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