I have never installed any package there since the system is very customized. In any case, I have put binaries or changed the configuration.

To do this, you must enter through ssh (user pi, password raspberry) and remount the file system:

'sudo mount -o remount,rw /root_bypass_ramdisks'

Under the /root_bypass_ramdisks directory you can edit something and it will survive the restart.

Good point for Oscar Fonseca

Moreover after the remount you can run

sudo chroot /root_bypass_ramdisks

and after run what you want in apt