(system) Custom fields for guilds

Using SSO, I want to autoprovision guilds for some/most of my groups in SSO and the manage the members in that guild (-channel) as well as autocreate some channels. I did not find a way to store an external_id for a guild, so that I can crossreference them with, for example, the ID of a group in my SSO. This field needs to be protected from user edits, so I don't want to "encode" this ID in the bio or name of the guild.

Either add a external_id model field for guilds or, more general purpose, add the concept of custom fields or tags for guilds (maybe same as user traits?) and have a way to make at least some of them admin-only (or via a permission flag) I'd then add an external_id to a guild, so my scripts can identify guilds and manage users/channels in them.