User services come with their own batch of design questions to solve,
which are pretty different from the questions raised by system services.
And most of these questions are policy-related, i.e. there's no obvious
answer but the answers strongly depend on what exactly the users want
and how the distribution usually organizes things.
It is difficult for s6 to answer these questions, which is why I
haven't tried tackling user services yet - save for s6-usertree-maker
which is a pure mechanism helper for users and distributions, and far
from a complete implementation of user services.
In a few years or so, when the s6 ecosystem is widely deployed as
a full-featured init system on several distributions and there are
experience returns on how they like to handle user services, I may
attempt to provide a unified solution.