Just an update on that: someone did fork my code and pushed it a littler further and cleaner :
https://github.com/TeLiXj/PhpbbSessionsAuthBundle
And another one trying to add Symfony 4 compatibility :
https://github.com/Macavity/PhpbbSessionsAuthBundle
Is there by any chance a way to make a pull request on phpbb repository or maybe to gain write access on the repository for the last maintainers ?
Thank you