A question for you. How well does advmess emulate philips CDI ?
My answer to your question:
I use Arcadia because this directory is automatically created when lr-mess is installed, so then there was no need to implement this in my script (no need to add cdimono1 as a system in /etc/emulationstation/es_systems.cfg) . Basically if the system cdimono1 is not in the es_system.cfg, software will not show up in emulationstation, that's the reason why.
Actually this script was made to bundle all the knowledge found on the internet to make CDI working without having to make an enormous effort every time to get it working.
The second goal should be to implement the knowledge in an official retropie-script. In this stage the rom-directory should be made like you said.