A 32-year-old ex-heroin addict with hepatitis C requires an urgent laparotomy for Rx of a perforated duodenal ulcer. The patient is undergoing treatment with naltrexone. The results of pre-operative coagulation studies include an INR for the prothrombin time of 1.8 and a platelet count of 85,000/cubic mm. Given this scenario, which of the following post-operative analgesic techniques would be most appropriate?