You could certainly talk to the gastroenterologist about the level and character of sedation you would prefer, but the exact medication reaction any one person will experience is not completely predictable. Two drugs were used for me. I think Demerol and something in the Valium class, but I can't remember what. I was not aware of pain or what they were doing, but yet I could hear my own voice calmly saying "ow, ow, ow, ow." And that's an excellent response and the reason they use the drugs they do. They don't want you to experience pain, yet they need you to be conscious enough to offer feedback about pain so they don't accidentally puncture your colon as they try to maneuver through tight places. The only thing unusual is that I could hear myself and remembered saying it. (The MD tried valiently to get through my twisted colon, upping my medication to triple the dose he normally gives, and yet my colon configuration would not cooperate, and all I got was a sigmoidoscopy instead of full colonoscopy).
Honestly amnesia really depends on the person and dosage of the sedative. I was prescribed a sedative to help me sleep when I was stricken with an unexplainable condition that Doctors are still baffled by. One of the side effects was suppose to be amnesia but I suffered no such thing. I am sure if you ask for a minimal dosage of the sedative lorazepam @ .5mg's or less you should be fine. Of course I am not a Dr. but that is what I was taking and could still remember everything that happened the next morning and it did not knock me out. Only a suggestion different people different reactions.
The sedative used which gives you amnesia is called Versed. Depending on the person it can cause amnesia even after you feel fine. There should be other combinations of sedatives they can use. Ask to consult with the anesthesiologist about other alternatives.