I did end up giving this a go yesterday, and I can confirm that I did get a snap to be produced but I would still have to do some reorganising manually for adwaita.so in the Adwaita-qt part and libqgnomeplatformtheme.so in the QGnomePlatform part as they still get installed under $SNAPCRAFT_PART_INSTALL/root/stage.
Hmm judging by the code it definitely looks like it should be using portals for the file chooser but this isn’t my experience. The file chooser doesn’t allow me access to removable drives indicating it’s not using portals. Being on GNOME I usually see the current application change to “Portal” which does not happen in my case.
The reason I want to include this within my content snap is so that Qt applications integrate better with the GNOME desktop. I also have plasma-integration included in the snap providing KDEPlasmaPlatformTheme.so as a platformtheme for good integration with KDE (the kde content snap also includes this). When using Qt apps from Flathub, of the selection I’ve used, they ususally use QGnomePlatform. Ideally I would use the content snap provided by the kde-neon extension, but it’s due to lacking certain libs for GNOME like QGnomePlatform and librsvg2 (problem related to this explained here) that have prevented me from adopting it as they don’t look at home on GNOME/GTK desktop environments.