Matt, thank you for your contribution. I already merged your pull request local!

I see, what you have more experience in writing ReSharper plugins. 
What do you think about redistributing targets file with project? Checkout my version in ReSharper.Target folder and csproj. This way user  without sdk can compile this solution for installed version of ReSharper (and linked to libraries which shipped with ReSharper itself). btw, if SDK installed Target file should link to appropriate version of it (condition attribute)