I am setting up a production system (will be deployed to multiple devices), based on Lubuntu. The system starts X, logs in a user automatically, and runs a full screen GUI application. That's all it really does. I'm using openbox as the window manager - in production I don't really need a window manager but its useful during testing and development.
For a number of reasons that I don't want to get into here, I would like to avoid lightdm (currently in use), lvdm, or any other display managers that perform functions I'm not using.
So, I'm looking for a display manager that essentially just needs to create a local session for a predetermined user. A greeter is not necessary, remote sessions are not necessary, the ability to operate on a read-only file system (particularly, /var/lib is read-only, although /var/tmp is not) would be a major plus.
Is there a super lightweight display manager out there that just does the bare minimum necessary to create a local session for a user? Alternatively, is it possible to remove the display manager entirely and just explicitly start a local session with a window manager?