WikiMedia editors have suggested that having an ISNI would be sufficient to allow a WikiData entry.
And MetaBrainz(?) projects are happy linking to artist images that are linked to WikiData as such images have been checked as having suitable copyright permissions.
This interview with Laura Dawson from Bowker (the ISBN distributors) gives as good an overview of ISNI as I’ve found.
But the specifics remain shrouded in mist.
I’ve found a MB artist Hannah Epperson who has been given an ISNI, and also got the results of an ISNI search for “Hannah Epperson”. (Google does not have ISNI numbers indexed )
"ISNI: 0000 0004 6307 260X
Name: Epperson, Hannah
Creation class: Language material
Creation role: performer
The muzieweb URL is “de muziekbibliotheek van nederland”.
Who, I conclude, is functioning as a source for, or a subsidiary of, Koninklijke Bibliotheek who is included in the list of ISNI registration Agencies.
Going by the search result, the standard that appears to have been used (by Koninklijke Bibliotheek) to judge eligibility is the existence of one commercially released album.
(The album is physical, not digital. It is not clear whether this was relevant to eligibility for an ISNI.)