I'd agree, we need more registration authorities to be included on the list. Maybe this information is something the Identify.org project could share with IATI as it says on their website that collecting reg authority info is an objective?
NB. We've only asked for one XI identifier to be issued for a CSO in the over 200 organisations we've worked with, and that was because there was absolutely no workaround. Usually there is a way and most CSOs and networks who want or need to publish to IATI are well established and will have some kind of official registration, even if it's at the province level or via a business registration service.