I added a QML file to my project, selecting its qml.qrc file as where to add it.
The qml.qrc file subsequently shows a new entry: "../../../" and then we hit the edge of the display area. It's actually using
../../../../../home/eddy/test/release/pythanim/HypoGrid.qml
as path. The relevant project's directory is /home/eddy/test/release/pythanim/
so simply listing HypoGrid.qml should work just fine (indeed, main.qml is in the same directory and is listed as such). For bonus comedy, the whole

* preamble could sensibly be omitted, since ../../../../../home is indeed just a long-winded way of saying /home/.

The net effect of using this preposterous path prefix (go to root and then come back to where you started) is that I get to chose between being unable to see which file it is (because all I see of its name is a ../../.. prefix) or waste a large proportion of my screen's width on having the Projects preview wide enough to accommodate this wide path (already somewhat indented, along with all of the qml.qrc details).