How to mirror and augment a git repository

Hello list,

I wish to mirror _and augment_ an upstream git repository.

              .--------.
              |Upstream|
              '--------'
                   |
          .----------------.
          |Augmented mirror|
          '----------------'
           /       |       \
   .--------. .--------. .--------.
   |Client#1| |Client#2| |Client#3|
   '--------' '--------' '--------'

Clients of the augmented mirror must have access to everything available from upstream but must also be able to collaborate on additional development branches not available from upstream.

Initial approach:

 Augmented mirror:

   $ git clone --mirror <upstream> upstream
   $ cd upstream
   $ git remote update  # regular cron job

 Clients (bare repo & worktrees preferred):

   $ git clone --bare <mirror> mirror
   $ cd mirror
   $ git config remote.origin.fetch\
         "+refs/heads/*:refs/remotes/origin/*"
   $ git remote update

This arrangement worked fine until I decded to run:

   $ git remote prune origin

on the augmented mirror and lost all the additional development branches the clients had added and shared amongst themselves.

I've tried running the augmented mirror as a plain bare repo, i.e.

   $ git config --unset remote.origin.fetch
   $ git config --unset remote.origin.mirror

but then the cron job (git remote update) is no longer sufficient in making all upstream activity available downstream.

So, how best to run an augmented mirror such as this?

If my initial approach was correct, is there a way to protect the additional branches so that ‘git remote prune origin’ may be run safely on the augmented mirror from time to time?

Any help/tips/pointers/suggestions much appreciated.

Sebastian