Instead of making an ArchWM-base as an individual package, why not an ArchWD (distro). A "distro" running within Arch. Not only getting rid of the rigid concept of WM (managing windows what is actually quite limited) but creating a new style of "WM" same as Arch did as a distro. The ArchWD still manages the windows but aims to distribute WM and DE related packages (modules and scripts). Not only Fvwm based packages but also others.
The style of upgrading the ArchWD system files and user contributed packages, will be same as pacman. The current name "pacwm" toolkit (confusing name) will be changed to "pactk" (package toolkit). To upgrade ArchWD and all installed user packages, simply run in terminal: pactk -Syu
What's the difference between doing this and, for example, creating a custom pacman repository of archwm packages? Keep it simple -- use existing tools. Dusty