That is easy - there is no such option
This is an all manual task
- add the necessary packages to the Packages-Desktop file
pamac search flatpak --no-aur - build your iso using the -x argument
- install the intended flatpak(s) on your local system.
- locate the files/folders added by the flatpak install command (
/var/lib/flatpak) - copy the added files to the $buildroot/buildiso/$PROFILE/x86_64/desktopfs
- build the iso using the -zc arguments
Be aware that the size of the ISO will increase dramatically - possibly why there is no option with buildiso to preinstall flatpaks.
I just ran a test
- installed flatpak support (nothing huge)
- installed 2 apps sublime text and sublime merge from flathub.
The size of /var/lib/flatpak exploded to 3.5G - the cost of providing preinstalled flatpaks should definately be considered.
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