Hmm, but the installed OS is not 100% the same as the livesys, or is
it? If not, it should be possible to add a "systemctl disable
dmraid.service --root=/path/to/os" somewhere, no?

AFAIK, they are 100% same. There's a hack, check your

/etc/rc.d/init.d/livesys

/etc/rc.d/init.d/livesys-late

They are executed every time during boot, but immediately quit if they detect they're not running on a Live image (using kernel command line). You can see them also here:

They are ugly, I think, and many improvements could be made there. But some Live image adjustments are possible through them. But those are just runtime changes for Live environment, they don't affect the installed system. If anaconda had a post-install phase where it would make appropriate changes to the installed system (and also ideally removed itself and those livesys scripts), that would be great, yes.