I see that Forum:Noble/Royal Titles, which didn't really reach a concensus, discussed the issue of the surname field for people who do not have a surname, since their families were entitled before surnames came into use.
As I see it, the principle of the issue we are debating are as follows: there is not "correct" answer to the question, since these people do not possess a surname in the same way as ordinary present-day people the English-speaking developed world do. So the discussion is actually about our contributors personal taste about how this should be resolved.
At present we have been following the original suggestion of Richard Tol, so that the House of Capet use the surname "Capet", the House of Welf use the surname "Welf", the House of Wittelsbach use the surname "Wittelsbach", the House of Wettin use the surname "Wettin", the House of Oldenburg use the surname "Oldenburg", the House of Habsburg use the surname "Habsburg", the House of Rurik use the surname "Rurik", the House of Romanov use the surname "Romanov", and so on. The present version is not totally consistant, and a lot of minor titles do not have assigned dynasty names.
A look at Familypedia:Surname statistics will give an estimate of how many paged will have to be changed if we change these rules.