Solution is hashtags + community name, not a "fedicommrc".
I was in FidoNet with a BBS in 1988 and on the Internet since 1990 first through a dialup vt100 connection to a Unix login access point for usenet and email before the Internet was available to the public. Communities are a special interest of mine. I started a particularly good one for Permaculture using a mailing list with email. I still run it. Noone should, for example, OWN the only permaculture community in the Fediverse. What are others who want their own going to do to gain users and generate traffic? call it permaculture2 or thatotherpermaculturecommunity or permaculture-general or permaculture-westernworld. Letting one group control any particular Fediverse named community is a really bad idea. Have you ever started and run a community (newsgroup, mailing list, subforum, fb group, google group, webforum, whatever)? Having one group control any named community is bad enough but everyone with an instance having to deal with a fixed list of communities for the entire Fediverse is absurd. You could easily have a multiplicity of communities with the same name but identified with hashtags for subjects they specialize it. This should solve your problem with community naming and with this no distributed list of communities for instances to carry would be needed. You could carry 6 different permaculture communities and each would be uniquely identified with hashtags alone. Make sure to code the software to feature those hashtags prominently along with the community name for ease of finding and subscribing to them.
Are you talking about a Fediverse version of a Usenet newsrc? Those who install an instance and want their own communities named as they choose would just do that and ignore any network-wide policy. Other instances can block them for doing that but they can simply connect with other like minded instances and form their own network and forget Federation. This is especially true when some gang of control freaks own a named community, i.e. permaculture. That is not going to fly. Disunity but independence within the Fediverse will rule.