It was decided (http://wiki.musicbrainz.org/Proposal:Artist_Type_Other) to add another artist type "Other" to the database. I don't think many changes would be required, it could just be added as (id,name) = (3,'Other') in the "artist_type" table.
I have patches prepared to adapt e.g. ArtistEdit.js, once it is added. Also the statistics page might need a little adaption (but only really small changes). Anything other I forgot?
If we need backwards-compatibility anywhere (ws/1?), "Other" can be replaced by "Unknown".