Ok, I might have searched a bit more before posting. I found the solution in the qtonpi bug tracker https://bugreports.qt-project.org/browse/QTONPI-11
, which involves installing the proper (32bit) version of that library. In Ubuntu 12.04, this did the trick:
Now I can compile the hello_world sample, and have it run on the RPi, straight from QTCreator. But, it doesn't display anything - only a white background, as most of the other included demos - even if I change the size of the screen element.
I am running on a TV via composite video.