Rustum seems like a variant spelling of Rostam, and I am surprised that someone would not have heard of Rostam.
Rostam or Rustam (Persian: رستم [rosˈtæm]) is a legendary hero in Persian mythology, the son of Zāl and Rudaba, whose life and work was immortalized by the 10th-century Persian poet Ferdowsi in the Shahnameh, or Epic of Kings, which contains pre-Islamic Iranian folklore and history. However, the roots of the narrative date much earlier.
Rostam allegedly lived in time of the legendary Kayanian Dynasty. Many historical persons have been named Rostam. When I first read about Rostam Farrokhzad (d. 641) I wondered if he was the legendary hero Rostam. I haven't heard of any Rostam who stands out to me as a great lawmaker or legal expert.
The name of Rustum doesn't seem to have been used by any famous lawmaker either. So unless Trollope used "Rustum" as a metaphor to mean someone who was as great a hero of the law as Rostam was a hero in battle, I have no idea what he meant.