I have not but for a better answer, you will need to say which African cichlids. There are tons of different groups and they all have their own likes and dislikes. If they are algae eaters they will be more prone to eating veggies than if they are bug eaters. Many have the idea that the fish who comb algae for the small bugs are eating plants when they really are just there for the bugs.
The peacocks are insectivores and are fine, but the mbuna might eat the water lettuce. It might be a good idea to add the plants in such a way that the mbuna do not think that you are trying to feed them with it.
Mbuna only specifies that they live near the rocks. It doesn't say anything about what they eat in nature. Old thinking was that they all ate algae but we know better now with more study. I will say that it does make them more likely to eat the plants than the peacocks but still not a very precise guess. For an example, the popular yellow labs would be expected to eat plants but other mbuna like Gephyrochromis moorii would not be interested nearly as much since they sift sand for their meals. For a lot of really good current info on what each is likely to eat, I would recommend looking at the profiles in this site: http://www.cichlid-forum.com/profiles/category.php?cat=2
I keep young carp and Koi which favor plant matter in outdoor tub's for use as bait during summer with water lettuce floating on the surface and water lettuce manages to grow faster than a dozen or two of these fish can eradicate it.
Guess it would depend on how much water lettuce you have, and number's of fish that are nibbling at it?:flick: