Yes, you can use Regasm on the assembly without being in the GAC. For example, I have mixed ASP and ASP.NET sites with .NET assemblies in the bin folder. Some of them are exposed to COM and are registered in situ from the bin folder.
Of course, these assemblies are unique to the application and are not used by other applications. This is not to say that this is impossible, you would probably choose a neutral place to copy such libraries.