I think that the answer may be that when Rashi (DH "Ela Beis") uses names such as Sheshbatzar for Shin Shin, he is telling us that even if there is no actual word that starts with Reish Reish, if a name or place is named with such letters in the beginning of the name then one is Chayev according to Rebbi Yehuda for writing the first two letters.

However, if you must have a word, I would think that based on the Pasuk in Vayikra (15:3), there should be such a word as Roro (Reish Reish Vav).

Kol Tuv,

Yaakov Montrose