I had time to do some more on this.

For putting the Qt5 libraries on device I didn't want to use the instructions on the URL in the first post, as the script does all kinds of stuff to the device I don't want. But all went well using this:

http://qt-project.org/wiki/Qt-5-on-Nokia-N9

Basically I did:
Now the bad news is that I could not get QtCreator to create debian packages using the new target. The basic problem is that the Qt5 target is recognized as Desktop target. You can compile your Harmattan executables using it, but you won't have the normal Debian building targets with it.

I tried to remedy it with all kinds of nasty stuff, like modifying QtCreator/share/qtcreator/Nokia/qtversion.xml. That didn't work as then QtCreator complained it was wrong Qt version. Apparently it requires Qt4. I also tried some beta version of QtCreator 2.5.0, but didn't have any luck with that one either.

I will personally wait now a bit if the next QtSDK/QtCreator would have Qt5 support for Maemo targets. If that doesn't happen soonish, I will look into using scratchbox (something I've tried to avoid, but apparently can't ). But if anyone gets QtSDK to build Qt5 harmattan debs, please share.