You can try to do this for Silverlight 3, and then reference it from a Silverlight 4 project.
I know that this will allow you to do this in Visual Studio, Edit: Confirmed, to work for the base function (shown below). However, library support is likely to be poor (or spotty) due to conflicting versions, but as a stand-alone library it should work fine.
module Module1 let testfunction value = value + 1
Rangoric
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