I got an answer from Asus Support and they confirm the only way is to remove the CMOS battery, but for 4 whole minutes, not just some 30 seconds.
In order to get to the CMOS battery, it is not sufficient to just unscrew the screws on the back of the laptop and crack it open, there are also four latches on the keyboard hidden behind F1, F6, F11 and Insert. This enabled you to remove the keyboard which is just the first step in a long process of dismantling almost the whole computer before getting to the CMOS battery.
I had to search for these latches for a long time and almost gave up when I gave a last googling. It revealed this tutorial on changing the keyboard on a similar model. This tutorial showed me how they look and thus I found them. They are almost invisible on the S101s black keyboard and when some dirt comes in there they are actually indistinguishable from said dirt.