I managed to make chromium use kdialog by staging the kdialog package and its dependencies, and by exporting "QT_PLUGIN_PATH: $SNAP/usr/lib/x86_64-linux-gnu/qt5/plugins" in snapcraft.yaml's environment section for the chromium app.
The problem with this approach is that it adds 60MB to the final compressed size of the snap. Some of it could certainly be excluded, but it will require some careful tuning.
I managed to make chromium use kdialog by staging the kdialog package and its dependencies, and by exporting "QT_PLUGIN_PATH: $SNAP/usr/ lib/x86_ 64-linux- gnu/qt5/ plugins" in snapcraft.yaml's environment section for the chromium app.
The problem with this approach is that it adds 60MB to the final compressed size of the snap. Some of it could certainly be excluded, but it will require some careful tuning.
This requires more work.