Is there any way for a Visual Studio 2015 extension to detect when a symbol rename occurs in a solution, and act upon it? Or perhaps this functionality is available in Roslyn somewhere? I found the IRefactorNotifyService in Roslyn, but this seems to be an internal class. Is there a public supported way of doing this? (Either in Visual Studio through EnvDTE or similar, or using Roslyn)?
You're looking for the existing IVsHierarchyRefactorNotify interface, which predates Roslyn.
In fact, the point of Roslyn's IRefactorNotifyService is to bridge to this VS interface (source)