In the old days of live isos such as Knoppix ran mostly from cd drives. These days most people will run from a usb drive in some form. Archiso has an option to allow persistence, this means anything done in a session will be saved to a named device. Simply at boot menu hit tab and add ‘cow_device=/dev/sdxN’ (x is used an example and N is a partition number). Once booted you can modify most of the systems settings including install packages (subject to a working network connection).
This is all fine but does have a few drawbacks. You cannot fully update a live system, more to the point you cannot update kernel. You may notice a longer boot time given loading of persistence folder/drive. Have noticed that if I run a newer version of iso current persistence files are not loaded.
A far better option if you want to run a arch system from a portable usb device would be to install arch and run from it.
Am sure it would not be that difficult to use something like this with ArchBang. This is far better way to go over a poor mans install and would result in a very portable, custom system.