I think of my favorite netlabels as devoted to a genre or related set of genres–webbed hand for ambient/experimental, darkwinter for dark ambient, wanderingear for field recordings, intelligentmachinery for experimental, earth monkey for quirky electronica, Birdsong for quirky material. So I think that if there’s an “umbrella” it usually is finding artists who work in similar genres.
Yet I agree entirely that internet release in general, and netlabel releases in particular, lend themselves to collectives and other voluntary groupings. To some extent, CCmixter can function that way. I’d love to see “schools” and “salons” and “collectives” and “movements” arise, because in each of those models there’s the sense of a group interest around a defined goal–which is a cool, non-annoying form of branding.