I've made it work a couple days ago:

1º download mysql essentials fron mysql site.
2º install everything on C:\MYSQL or some path like this, so you wont get headaches about spaces in paths
3º build the plugin exactly as this guide says
4º add in your .pro file in the QT section the string "sql" (without quotes)

do exactly that and you should be ready to go!

ah... btw, on the guide, ignore the "MinGW tips" section