Yes, and if you were going to do it, I would forego the lemon juice or vinegar, otherwise you'd have a big, foamy mess. Baking soda by itself would probably react with your pouch contents as it is.
However, I think you are misunderstanding the purpose of sodium in other enemas, such as Fleets. Those are concentrated salt enemas, and are not specifically for "flushing out," but work by drawing water from the intestinal wall by diffusion (simple chemical physics). Sometimes too much salt can be absorbed in the process, and that is why those things can be a problem for folks with kidney failure or hypertension. Anyway, the water moving into the bowel softens stool and induces a movement.
We usually do not need that (we have plenty of liquid in there and hard stool is not an issue), and that is why tap water enemas are recommended. Rinses things out, just like flushing the toilet! A little salt or soda won't hurt anything, but I doubt it makes much of a difference, especially when it is in there for less than a minute.
Jan