The only real requirement is global visibility and, with IPv4 today, it is granted to an ISP with multiple external connectivities, which is not necessarily the case with multi6.
Actually it doesn't work like that in IPv4 either. You can get an AS number if you multihome, but for your own address block you need to show immediate use for a /22 (in the RIPE region).
-- leo vegoda RIPE NCC Registration Services Manager