there is an alternative missing.
you can change a customizing activity to be a master data activity, and you do not need to have the PRODuction system open for customizing..
this is explained in OSS Note 135028 - Transfer IMG activity to current setting
However, I have made my experience in the past when I joined my current employer. In this system the purchasing groups were maintained in production system, which I did not know, and some key user did not know too, as they asked me to add a new purchasing group. I did this in the development system and transported to production.
And it happened, that I added a purchasing group in development that was already occupied in production, so the entry in production got overwritten with my transport. So for a couple of days 2 buyers worked with the same purchasing group, they did not even realize it, as nobody is really looking at a PO form if a system is live for many years. This caused some trouble with statistics and as well with communications between vendors and us.
A second experience was made by the fact that people had deleted purchasing groups, after buyers had left the company. But SAP validates the existence of purchasing groups when archiving purchase orders. So I had to recreate all deleted purchasing groups before I could continue with archiving.
I think that this experience is reason enough to not give this customizing activity into user hands.