Something else to consider when feeding silkworms other greens is the nutriticional content of the food and thus, the silkworms. Alot of what makes silkworms so good to feed reptiles is their nutritional content which they get from eating mulberry leaves. Feeding them lettace, I am assuming, will drastically change what the silkworm has to offer as a food source.
Testing to see what nutritional value a silkworm raised on kale or other non mulberry green would have to be done by a laboratory specializing on determining the food quality of food. Or so I assume.
Personally, though it does cost a bit extra and there is some minor inconvenience involved, I would rather feed mulberry leaves or the silkworm chow to silkworms and be assured they are just as healthy as they can be for my reptiles. However, I would be interested in learning if silkworms can be fed other mixed greens with high nutritional contents and still maintain their value as a food source.