I don't know but I would guess that the editor itself, when you creating a gui, will create elements from QT5 classes.
When QtCreator is so intelligent and uses uic on the design.ui file from madde fremantle then it could work. ( I need to look in the makefile)
I don't know if there is a big difference between qt4.7 ui-files and qt5 then maybe the uic command can not handle these ui-files from qt5/QtCreator3. Also you should use only classes which knows qt4.7-maemo.

In principle you don't need QtCreator and you can use Madde with the console tool. So everything what you need should deliver Madde it is a matter of the configuration of QtCreator and the question about ui-file compatibility.