I managed to get the 90 day trial version of Visual Studio 2008 and figured out how to build x64 versions of the plugin. http://www.farfarer.com/blog/2012/06/12/unity3d-tangent-basis-plugin-xnormal/
Sorry this took so long, the Visual Studio project ended up with wrong settings somehow having set it up in 2008 rather than Enterprise but I finally sorted it out (it's all complete jargon to me). Also discovered the versions I'd released previously were still in unoptimised debug mode, so hopefully this'll be a little…