Recently, in the Species War thread, some things have become clearer about taking over sites.
1. If you successfully take over a location it will not show up for its original civilization while in Dwarf Fortress mode and choosing an embark. (the dorf icons displayed when you have a particular civilization selected).
2. If you successfully take over a location it will no longer be possible to embark an adventurer from that site.
3. The XML data export for Legends mode does not quite work properly yet. However, if you have taken a site over the LegendsViewer utility will show something like ...
In 52, midsummer, (Malachite, 14) UNKNOWN HISTORICAL FIGURE linked to Tellwealthy.
In 52, midsummer, (Malachite, 14) UNKNOWN HISTORICAL FIGURE linked to Tellwealthy.
for the actual site detail and in the world summary there will be a note like ...
Site ownership conflict: Tellwealthy. Actually owned by The Early Thorns
(assuming you got the group name 'The Early Thorns' when you claimed that location).
As seen in Species War it is entirely possible to claim sites (the Elves claimed every last human site leading to the effective fall of the human civilization). After everybody has taken their adventurer turns I will check the legends data to confirm how many (if any) sites each adventurer has successfully claimed.