I've got an older assembly (Office XP PIA) that appears to target the .NET 1.0 framework. The assembly has been registered and is visible in the GAC (as well as the Win32 registry), but it doesn't appear in Visual Studio 2008 when I choose "Add Reference".
Does Visual Studio support referencing older assemblies?
Yes, I believe so. You would have to manually browse for the assembly, however.
The following link,Common Pitfalls During PIA Deployment and Installation, may provide the information you are looking for.