OK , I think I've determined what was going wrong. rather than giving each generation of mods a new assembly name I'd been utilizing a common name and simply deleting the previous generation from the GAC and registry. This apparently does not provide for a clean registration of subsequent assemblies. Once I'd registered the usercontrol assemblies on a clean machine everything worked fine , re-registering on the original machine w/ a new assembly name seems to confirm that the registration of my previous attempts had be corrupted.
so the MS advisory is basically correct . I'd advise generating a GUID rather than rely on regasm. Also , the control must inherit from UserControl . And if you're attempting to revise an existing GAC assembly to make it viable as a plug-in , DONT - register under an original name.
