Would you recommend adjuvant capecitabine and radiation in addition to adjuvant FOLFOX for a patient with resected pT3N2 rectosigmoid adenocarcinoma with other high-risk pathologic features?

As with all rectal tumors, those that are labeled as "rectosigmoid" should be reviewed in a multidisciplinary tumor board at a center with deep sub-specialty expertise. In general, radiation is provided for rectal cancers--unlike for colon cancers--because of the increased risk of local recurrence and the morbidity and mortality that follow such recurrences. The lower a tumor (that is, the closer to the anal verge) the more likely such local recurrences are. So, high rectal tumors likely benefit...

No, there is no evidence of any local control limitation or survival benefit in colon cancer unless there's a perforation. There is no indication for radiation in this case.