Poettering: Factory Reset, Stateless Systems, Reproducible Systems & Verifiable Systems

If you have a system with local disks, which is what I think you're talking about, then one solution would be to write a udev rule. Detect a RAID controller, launch a script, probe the controller and its drives and create a mount unit for it at /raid or /var/data or whatever you like. That way systems with locally attached storage get it set up, but it doesn't interfere with other systems that just have a boot drive.

Or just do it with Puppet or whatever configuration tool you're using. A stateless system isn't very useful until it has grabbed itself some state after all. The configuration doesn't need to be in /etc, it can be on a network service.