This shouldn’t be a distribution feature - there is too many variables which will make it a hit or miss.
So I am thinking outside the box now.
When you install your system using online - all packages are stored in your pacman cache.
You can use the utility repoctl to create your own mirror database using these packages.
You can even use repoctl to run a local mirror.
Then point the booted system’s pacman.conf to your local mirror.
Sample script collecting package lists and configs