UPDATE: Based on the discussion held on 2023.02.07 Weekly Meeting with the Slicer core devs, it seems reasonable to try approaching the Slicer flatpak under a superbuild approach rather than a non-superbuild as I initially proposed. Reasons: (1) some of the SystoleOS patches may take time to land on the Slicer code base, some might not even make it; (2) the distributed Slicer should be the same (or very similar) regardless of whether this is a flatpak or a redistributable linux binary, as of today this a superbuild-based.
@luarpy and @RafaelPalomar have started putting together a Flatpak manifest generator for Slicer (this is needed to set the right versions for the dependencies in the manifest) in here: GitHub - RafaelPalomar/Slicer-Flatpak: Flatpak for 3D Slicer. The final manifest will go on its own repository (so far, GitHub - RafaelPalomar/org.slicer.Slicer: Flatpak repository for Slicer). It is still a non-usable work-in-progress, where we try to reproduce the suggestions above by @jcfr.