Hm, it already should work with no network. Where things might go a bit awry is where there is a network, but there is no archive mirror on it. In these cases it’s probably better to not configure the network during install and put the desired config into place in a late-command.
Good timing, just a few days ago I put together a thing for doing pretty much exactly this: A tool to modify live server ISOs. It’s still a work in progress (it seems something isn’t quite right yet in the process of regenerating the apt indices for the pool on the ISO) but it’s definitely something you should look at 