That’s a very nice and promising article @aertmann, thank you ! I will most probably go through it in practice since I have a use case for users in dynamic multisites.
What do you mean that your approach only supports Neos 2.x ? What would cause compatibility problems in Neos 3.x ? You mentioned to ask you or @bwaidelich about it, so that’s what I’m doing now 
Also, @kdambekalns, I would be very interested in those packages, where can I find them ?
UPDATE:
The mentioned packages are probably the following:
As noted above, they are not “officially announced” yet, I’m posting the links in order to be able to find them again later.
Thank you for sharing and thanks @christianm for helping me find them !