Why would that be a reason not to add it to packagist? I am not asking you to add it to your own phpBB. Besides, it is not necessary to change the phpBB composer.json to install extensions to it and I don't intend to do so.
You can also create a separate composer.json, which, if it depends on the composer installer package as well, can install extensions to the correct directory automatically. You can even download phpBB itself using composer AND put the extensions in the correct spot without ever touching the phpBB composer.json. Here
I have posted an example that could be used. It goes into the folder above phpBB and will install a phpBB to a folder called "community", including several extensions.