One solution may be to make the local csproj file writable.
In this way, when you'll add a temporary test file to the project, VisualStudio will not checkout the project file anymore. When you're done playing with the test file, don't forget to do a Get.. operation on the project file and select to replace with the version in source control (this will undo the temporary changes to the csproj project file).
But I don't see what's wrong with letting VisualStudio to checkout the csproj file...
You can have multiple checkouts enabled in the VSS database (in Admin, Tools/Options/General page), then each developer who adds a temporary test file will checkout the csproj file, and in the end when they are done they can simply use UndoCheckout to go back to the initial configuration.
Alin