Instead of a clamp, have you considered a lopez valve? Similar to a stopcock device, it allows you to close off the tube between syringe-fulls of liquid, and your hands are free, and you don't get any leaking back or spilling from the tube while you are manipulating the syringe. Most hospitals/E.R.'s/etc will have them, they may also be available in hospital supply stores. It is what we use in the ICU when the patients have "foley"-type G-tubes, and they sure save a lot of time and hassle.
Andrea