Glycosyltransferase involved in the formation of rhamnogalacturonan I (RG-I) oligosaccharides in the seed coat mucilage, which is a specialized cell wall with abundant RG-I (By similarity). Transfers the rhamnose residue from UDP-beta-L-rhamnose to RG-I oligosaccharides (PubMed:30082766). alpha-D-galacturonosyl-[(1->2)-alpha-L-rhamnosyl-(1->4)-alpha-D-galacturonosyl](n) + UDP-beta-L-rhamnose = [(1->2)-alpha-L-rhamnosyl-(1->4)-alpha-D-galacturonosyl](n+1) + H(+) + UDP Glycan metabolism; pectin biosynthesis. Belongs to the glycosyltransferase GT106 family.