- In addition to post-config.d you can also use (if it fits your needs) firstboot.d and pre-config.d (I guess the names are self explanatory). Ref. LINK
- One option would be to create custom default config - modify /opt/vyatta/etc/config.boot.default ...to start the auto-provision. This could be done by creating custom firmware that would be flashed onto the system. What you need are squashfs tools, specifically unsquashfs and mksquashfs. Some early attempts ref. LINK
- As for persistent storage that survives reboot, everything in /config survives reboot. On upgrade content of /config is copied to new image. /config is actually in /root.dev/w and config from the other image is always in /root.dev/w.o. Knowing this gives you access to the config from one image to another in case you forgot to copy something over. I also believe that any custom folder in /root.dev/<folder> would survive indefinitely (I've not tested this though ...needs confirmation). Mind space on ER-X and ER-X-SFP is tight. On models with USB stick such as ER-L and ER-5-Poe you can replace the stick and do whatever you want.
I'm planning to do my own "call home auto-provisions" using some very basic script put in /opt/vyatta/etc/config.boot.default and resquashing the image. ...when I get some time, maybe over xmas 