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Option at the bloodline registry to add a surname to public record. Would let people see the most prominent bloodlines and where they came from if you add the city of origin option (rogue being another option for no affliation). Some bloodlines might have an even distribution over two or more cities so you could have a setting for 'origin' and for 'current base'.
The only issue in registering members would be people who are lying, so there would have to be a limit per person of claiming membership to no more than 2 families. You could of course have a single head of the family but in large families that could be hard to determine, so probably a referendum type confirmation from active family members, wouldn't take long for people to confirm or deny. If anybody gets a denial there'd hopefully be a reason for it and require administrative validation (and possibly revoking the voting rights of the troll who said no).
tl;dr - implementation got a little more complicated as I went along, basically create a surname registry
- alphabetical, number of members, city, etc.
I like my steak like I like my Magic cards: mythic rare.