Shepherd user services that run on startup?

[Ben Weinstein-Raun]

Hello! I'm fairly new to using Guix System, and I have a lot of questions.

One is: Is it possible to run a shepherd user service at system start, 
rather than at login?

My guess is that this is at least possible, by defining a system-level 
service that starts a shepherd for each user. Would that work?

Anyone have tips on how to go about building this, if so? Especially: 
What's the easiest way to ensure that a guix service knows the list of 
users-with-login-shells on the system? And easiest way to run a 
`shepherd` instance as each such user on startup? I'm working on writing 
a service definition, but as this is my first from-scratch service (guix 
or shepherd), and one of my first lisp projects since reading SICP 15 
years ago, it's slow going (even with the reference manual and source 
code of other services).


And, of course, is there a simpler way?


(I also asked this question on the r/guix subreddit, but haven't heard 
anything yet, and figured I'd ask here too)