The ICD-9 index under polyp, rectal states adenomatous are 211.4. Non-adenomatous rectal polyps are 569.0. Everything I find indicates that even though hyperplastic polyps are benign, they are not adenomatous.
familial adenomatous polyposis:
"a disease of the large intestine that is marked by the formation especially in the colon and rectum of numerous adenomatous polyps which typically become malignant if left untreated"
211.4 is appropriate