I have one hidden KCM already. It was requested I make the super awesome qtquicksettings module hidden. It uses a little hack to make it findable in krunner, but not listed in system settings by parenting to a group, then hiding that group.
I have no objections to things that are only useful to devs being in a hidden KCM. However, I fear it will end up becoming some sort of hidden plasma-tweaks, where we dump general aesthetic/behaviour settings in here which we think are too niche for the main UI. I don't want that to happen.