For those who have a very alkaline or calcareous soil, phosphorus is not very available, and soft or hard rock phosphate will not easily become soluble. It can be added to a compost pile instead to become incorporated into microorganisms, which thus makes it available to plants once the compost is spread.
I would assume that a worm bin would do a really good job of this, and the phosphate might be good for the worms too.
In order for worms to be able to "eat" rock phosphate, it first has to be broken down by bacterium and then worked on by fungi hyphae that are attached to the plant roots.
This makes it not so useful for worm bins since it has to go through a minimum of two breakdown steps prior to the worms being able to digest it.
If you want to give it a trial then by all means do so and keep a note book, you will also need the laboratory capabilities to test the worm poo for phosphates.
Worms do not (contrary to most beliefs) process soil, they do process microorganisms and "red wigglers" do eat vegetative matter, much more so than "earth worms" who live much deeper than the wigglers.
Yes worms need grit the same as chickens and other fowl. But this is not going to be broken down when it passes through the worm gut.
I will make you an offer here. In two years I should have time to do some of the chemical and biological testing you mention.
If I can get to that project I will publish the results here, in fact I will post those results in this very thread, making it easier for folks to find and utilize.
Below are links to the current studies on the earthworm gut and how it processes foods and what it processes for food.