Thanks for your work packaging ParaView for Flatpack!
You are correct that HoloPlayCore contains proprietary libraries that are not open source. As a result, HoloPlayCore source code is not available for packaging. We do have approval from Looking Glass Factory to package ParaView with the Looking Glass plugin for distribution on Flathub, subject to whatever restrictions Flathub may have on applications with non-open-source components, so please feel free to include it in the Flatpak.