That sounds like a good idea and there is no doubt it can be done but for the time spent installing there is no real benefit. Why?
Because the Manjaro ISO is build of - at the time of ISO creation - four filesystems.
Those file systems is unpacked into memory when the ISO boots.
The Calamares installer does the usual preparation of the system and when finished package collection then on the user's final accept a selection of the file systems is unpacked to the target partition(s) mainly the root and desktop file systems.
So an optional update of a system to the latest set of packages will take additional time to download and install because the aforementioned file systems were packages available at ISO creation time.
So no matter where it is done, it requires additional time.
Another option is for maintainers to rebuild the ISO a few days after a major snapshot has been made from testing to stable.
The ISOs would then be semi rolling and new installs would not have to download more packages than what has been released during required security updates or important bug fixing.
I may not have understood the OP correct - so trying to follow here - the suggestion is for the live ISO to - on the user selection - behave like Architect with the exception that the profile is installed instead of copying from the ISO.
Wouldn't that be to duplicate Manjaro Architect into Calamares.