Hi David....
I just started thinking about this five minutes after my last post, and I though... "how does it work for the core UserOnline Module?"
So, yeah the values are in a database (has long was you have UO active in your host settings and ofc the related Scheudle job)
It would be relay nice to see SQL integrated to DUD... Its realy the only way this would be possible.
Forms And Lists is another core Module that uses somthing similar, tho its more simple... you can select "User.IsOnline" or something like that from the DataTypes, but I think that value also comes from the UO table.