./QtCreator/lib/qtcreator/plugins
Looks to me as though this would be the most logical place to put it. What else is in that directory? On my Windows installation of Qt 4.8, the Designer plugins are located in /plugins/designer under the Qt root directory. Things like arthurplugin, qwebview, qdeclarativeview, among others.

I am not using QtCreator however. I use the qt-everywhere-opensource-src-4.8.0 distribution, and rebuild it all using the VC++ compiler.