I can not say for sure without running exhaustive lab tests, but my gut feeling is that castings are pretty much castings-AS FAR AS NITROGEN IS CONCERNED. Probably with small, insignificant variance. I think where you might see a difference is the trace minerals that might be provided from a varied diet.

An animal breaks down protein into amino acids then reconfigures those aminio acids into it;s specific proteins. Of course excess is excreted if not utilized for energy. i don't know how much energy need a worm really has.
Alot of the protein a worm gets is not necessarily from it's food like a human's is. Instead it comes from the microbes that are breaking down the carbohydrates and using basic nitrogen sources -more like a cow.

On a side note here, a piece of beef from grass fed cow is just as good a protein than that from one fed a fancy diet. Cattle are fed supplemental feedstuffs to get them to grow a little faster or put on more fat- because fat is what gives meat its taste.

Excess feed always appears in waste. My own idea is to make the process as simple as possible. Get rid of my garbage with as little processing/energy expenditure as I can. And certainly I will not BUY anything for the worms.

"On a side note here, a piece of beef from grass fed cow is just as good a protein than that from one fed a fancy diet. Cattle are fed supplemental feedstuffs to get them to grow a little faster or put on more fat- because fat is what gives meat its taste."

Bad example. Both grass-fed beef and the ultra-expensive Kobe beef have superior fat profiles versus grain-fed or grain-finished beef. (From a human eater's perspective.)

"Is there, in any other way, NPK or otherwise, any significant difference in the resulting product or does it all come out the same in the end?"

The reality is that nutritional science is still very much in its infancy, and what we do know is poorly correlated to plant nutrient uptake. NPK levels could certainly be tested in a lab. Plants could be tested for sheer growth on one type of vermicompost versus another, or resistance to specific diseases, for example.

Measuring what those precise differences are, however, is a whole 'nother squirm of worms, especially when you get the rest of the food web involved.

I don't think that at this point that more could be said with certainty -- definitely not anything like whether or not different worm diets ultimately lead to healthier produce (from a human eater's perspective.)

Which leads us back to gut, intuition and anecdote. My gut says there probably are differences, but the differences will likely lie in micro-nutrients which we don't really understand anyway. And since yours are wild scavenging worms, you couldn't really control their inputs... or what else is inputting into your system.

So my conclusion is the same as pjames: my worms are there to turn waste into a useful product. I may choose to exclude things from their diet (like pesticides) but I don't worry about the small stuff because I just don't think we have the science to prove anything one way or another.

My comment about the fancy diets included the kobe. What I said was "just as good a protein than that from one fed a fancy diet". And that is because the excess fat is what gives beef the higher rating.... even though it may not be superior as far as nutrition goes. I think you will find that grass fed beef tend to be leaner... otherwise why would meat packers use feeding lots to 'fatten' cattle before slaughter?

And you are right nutrition is still in it's infancy... but it is a REALLY late baby... They thought they were in their infancy 30 some years ago when I was in college.. Goes to show you that research is ongoing and there is always improvement and changes in what is "right or wrong."

I don't think we will ever prove alot with worm nutrition. As a scavenger they do great. But the worms themselves are not a significant end product. A slightly better bait worm is not going to make a difference. Nor do I think in our culture earthworms are ever going to be a part of a human diet.

Our job is to figure how to best manage the worms to accomplish their mission. Which, again, is to process garbage.

My references to "corralling" worms versus "free range", etc. were only metaphorical. Though allusions to the cattle industry or livestock in general, it isn't my intention to use them in the same way.
I thought my original post was clear but discussion of kobe beef, lean vs fat, effects of diet on flavor of flesh, whether "earthworms are going to be a part of a human diet" etc. leads me to believe there may be some confusion. To eliminate any further possible ambiguity and get the discussion back on track, the end product (pun not originally intended..) the quality about which I'm inquiring is the castings, not worm meat (wormeat?).

Tests have been done growing plants in castings produced from eg different manures, which showed that pig manure was better than horse manure which was better than cow manure etc.

So yes, there are differences depending on what the worms eat.

It only makes sense. The worms cannot ingest and excrete anything that is not fed to them. Just like you cannot spin straw into gold, a worm cannot excrete more nitrogen than it is fed. The quality of the castings is determined by what the worms are fed. This isn't alchemy.

pjames - you are right, I missed that protein reference

actionclaw - You are asking about the quality of castings, but they are not an end product in themselves. People use them for improvement of soil tilth, fertilizer for both ornamental plant and vegetable/fruit production, etc. Any definition of quality has to measure how good they are at the use to which they are going to be put. The vegetables were just an example.