I've been looking into raising insects for feed - for animals or humans. And I stumbled across a method of harvesting fairly large quantities of worms fairly quickly. It is called worm grunting / worm charming / worm fiddling, and apparently has been done in the southeast of the United States, and in the UK. The basic premise is to make sounds in the ground which sound like moles digging, triggering the worms within a radius of multiple meters to flee the soil and come to the surface. They can then be picked up en masse. Traditionally done in the forest, the best gathering areas are where there has recently been a burn, so the ground is clear and the worms are easy to find.
In some of the areas where this is/was practiced in the US, the purpose was getting worms for fishing bait. Due to overharvesting, it was eventually regulated, and nowadays very few people do it commercially because it is a tough way to make a living.
Here's what I'm thinking. Earthworms (like many insects) can provide an extremely nutritionally dense form of animal protein. In a permaculture setting, the ground should be teeming with earthworms. In the same way that cattle can be used regeneratively, even though some are harvested, couldn't we use earthworms as regenerative soil workers and periodically harvest some of them? One could dig up some soil, estimate the number of earthworms, and based on the lifecycle length do some calculations about what a sustainable harvest rate would be. I bet it is pretty high.
The professionals use a stake pounded more than a foot into the ground, and rub the top with a metal bar. But if you want to do this just with things growing in your garden or woodlot, here's a very simple way to whittle a stick tool to scare out the earthworms:
Here is a guy using a commercial version of the stick tool, with some tips. He says it is a lot like fishing, in terms of getting a sense of when, where, and how to get the worms.