Sodium Citrate solution was used in the Index (J&J) disposable enema (probably been off the market for 40+ years now). Not recomended in large doses, the pH of the solution would be quite high.(Sodium is a strong base, citrate is a weak acid).
Magnesium Citrate in an enema would probably not be especially effective. Magnesium Citrate to a certain extend requires that the digestive process break down the Citrate. What ultimately counts for oral administration of most magnesium based laxatives is how much magnesium is actually present.
Magnesium Hydroxide (MOM) is about 35% magnesium by weight.
Magnesium Citrate is about 11% magnesium by weight
Magnesium Oxide is aboug 60% Magnesium by weight(Phillps Caplets).
Magnesium Sulfate taken orally (Epsom Salts) is usually magnesium sulfate heptahydrate, and the Epsom salts is about 10% magnesium by weight.
From that information you can calculate equivalent doses of Magnesium based laxatives.
Magnesium consumption in excess of about 500mg can be expected to produce a laxative effect. The more magnesium you take, the greater the laxative effect. If you have kidney disease, magnesium is NOT recommended. Considerable amounts of magnesium can be absorbed from these products. Normally the Kidneys are very efficient at removing excess magnesium. Bad things happen if the serum magnesium level gets too high, hence the advice about avoiding magnesium products if you have kidney disease.