#5 Make ro branch temporaly to rw branch with highest priority

[MasterQ]

Hello,
i have a box with a linux installation on a sdcard. To protect the box from changes and to protect the sd-card (without wear-leveling) from many write-cycles I use aufs with to branches:

1. ext4 ro mounted partition /ro
2. ramfs partion on /rw.
   I have mounted both and merged them with aufs that works fine.

I want to give this box away. But it is possibile that i have to give support and need /ro branch to be writeable.
For example for keeping log files after system crash or make a system update.
I see, the second example could be made in chroot enviorment.

Teoriticaly something like:

1. remount the /ro with -o remount,rw
2. mount -o remount,mod:/ro=rw /
3. mount -o remount,mod:/rw=ro /

Should solve the problem, but of cause /rw will be allways busy.

So what i like to ask is...
Is there something like:

1. remount the /ro with -o remount,rw
2. mount -o remount,mod:/ro=rw /
3. Tell aufs so save new files to /ro

Many thanks for reading an helping!