Well, it is difficult to say, whether Duryodhan was anti-casteism or not.
If I am not mistaken, it is mentioned in KMG, that after Duryodhan made Karna the King of Anga, he felt relieved. So, this step was certainly to secure his side by appointing someone equal to Arjun.So, atleast, in the text, I have not come across any fact that proves that Duryodhan ever did anything for the lower caste.However, many people claim that Duryodhan is worshipped by some tribes. Maybe it is true.But I would like to point out that, many characters of Mahabharat, apart from Lord Krishna are worshipped in many parts of the country.In South India, if I am not mistaken, then there's a whole cult for Draupadi. There are temples of Draupadi Amman, guared by statues of none other than Arjun's son Iravan. Even Subhadra and Balram are worshipped by people of Puri, Orissa. Even Hidimba is worshipped in Manali.So, I think, it is too far-fetched to make any claims, based on this.However, this is my opinion. Many people may not agree.