I suppose the domain field can be used a workaround. So lets do the assignment client an update to the data web when UserActivityLogger::close is called, setting the domain field to Offline.

When the user is teleporting the domain will be updated to the correct domain in the interface client Application::updateLocationInServer.

In MyAvatar::goToLocationFromResponse there is an check if the domain is set to 'Offline' thus indicating the user who is the teleport to target is Offline.