Apart from the ones just listed, looking specifically at the 20th century, the most notable group have to be the Conservative Revolution thinkers in Germany. To my knowledge, a good number of those were aristocratic. There were also a lot of aristocrats involved in the SS. In Spain, there were also aristocratic rightists - both conservative and modernist. The Carlists were a more traditionalist movement, whereas de Rivera's Falange was more modernist. In Italy, there was Gabriele D'Annunzio, the grandfather of Fascism, although perhaps he should not count, since to my knowledge his title was bestowed to him later on in life. In Britain, the only notable rightist aristocrat I can think of was Edward VIII, who was more on the modernist side, however, since his sympathies laid with National Socialism. In Austria there was also von Schuschnigg, who I believe was an aristocrat. Southeastern Europe also had several authoritarian monarchical regimes during the interwar and second world war periods. The Romanian one is probably the most well known on DAR, because of Carol I's actions in assassinating Codreanu and establishing a short-lived, astroturfed pseudo-fascist regime.

I have to confess, though, that I am not very well read on 20th century aristocratic traditionalism. I only know of the more public facing groups and individuals, like Evola, de Rivera, the Conservative Revolution etc. I am sure there are probably a lot of other important individuals who moved in private circles.