OK, so I've made a few experiments and found that libqtm-11-multimedia on its own was not enough. MainPage.qml wants to #include QtMultimediaKit which was available after installing, but MainPage.qml could not find it. The bare minimum I had to install to make it work was libqtm-11-declarative, but that did not save me much: with dependencies, it meant installing 16 packages instead of 17. So it looks like the best choice is to install libqtm-11 in its entirety: the difference is negligible (a few kilobytes) and the benefits far outweigh the extra space.

The only trouble is that libqtm-11 in extras-testing is broken (does not contain all the dependencies) and I had to install the one from extras-devel.

Finally, all this effort only to play a game that only works in portrait? Hardly worth it