Flex weights are nothing new and used by quite a few carriers for many years. We have around 5 to 7 different MTOWs that we can change as we like on the go. We get a few more passengers? Just change the MTOW on the loadsheet and off we go. That is indeed legal and approved by both the LBA (our governing agency), the DFS and eurocontrol. However from what i hear via the grapevine Eurocontrol will try to get rid of that system and rather use one fixed MTOW as that generates quite a bit more cash for them.
Nowadays the MTOW is changed by centralized load control (a third party contractor) most of the times, not us pilots anymore.