With no factual knowledge or expertiese I feel free meal worm castings would have the same value as free starbucks coffee grounds. I would put the meal worm castings into a bird feeder the birds could easily scatter the stuff to look through it. You might attract some cool birds. Around the feeder the ground would benifit so plant something to grow wild. Putting some into the worm bins should be fine too. I would not try to make beer out of them. :-) Free is good. Organic material is good. It probably does not have a high salt content. I do not think the bugs will survive out of their natural habitat.

Years ago i had a pet store and also my own private collection of reptiles. I used to raise mealworms so i could have freshly hatched mealworms to feed baby herps that were just too small to handle the ones from the commercial growers.

The castings went into my houseplants and they did great. So if you get them for free, go for it.

Now as far as which is better, mealworm or earthworm castings, my intuition says earthworm. My reasoning goes like this:

The digestive process is different. The mealworms eat 'meal' a mixture of grains etc. They convert the sugars and protein pretty much directly into their own protein and fat.

With the earthworm, they can do the same to some degree if the particulate size is small enough. However they mainly rely on microbes to break down the cellulose and macrostarches of their foods into sugars and amino acids in a form they can absorb. The resultant feces is filled with microbes that seem to have an extra beneficial effect in garden soil. You can look up some studies on this and comments of mucous-like compounds that offer surfactant effects, etc.