It seems to be some confusion if the GacUtil is the correct way to get assemblies into GAC at all.
MS seems to be pushing an avenue where one should use msi installer projects to get assemblies into the GAC.
Trying this approach shows why it is not acceptable - running an install script adding eco and modlr assemblies to GAC (so that our VS-plugin has a chance to find them) takes about a minute or two. While running GacUtil on the same set in .net3.5 takes 1.5 seconds.
All in all we are close to be able to release ECO for VS2010 but the little issues like a missing GacUtil is stopping us.