Is this something that has ever been done successfully?
Manure is quickly claimed in my region and I refuse to buy horse crap pellets by mail from a foreign country. The postage fee would be shitty

I have an abundance of wild grasses.
I have a big pile of nearly dry Brassica straw from this years seed crops.
To make liquid organic fertilizer I tightly pack wild grasses and edible weeds into large buckets, fill them with water, and let ferment for 1-2 weeks [dilute liquid to 1/10th strength for use] and a byproduct obviously is a large mass of fermented plants a horse would eat. It smells like horse manure and my dog gets excited and rolls in it
I also do the above with biomass [often including sticks] chopped into 2 cm sections and mix the solid byproduct with 1/4 its volume in used and washed potting soil, and then compost the mixture for 6-12 months to make more potting soil. I've seen mushrooms grow in the tubs. I have much of this already prepared.
I also have an outhouse only used by a vegan that gets a high fiber, low salt diet.
Very little goes to waste here

Would any of those resources, perhaps with the inclusion of some materials from the supermarket, serve to form a substrate from which I could grow P. cubensis mushrooms?
Yield would not have to be high, since the substrate would mostly be made from free things I have in large volume. I just want a second source of mushrooms along side my old shotgun terrarium and some experience on techniques beyond BRF cakes.

I can not speak to the cultivation of cubensis- but I wanted to mention a good read called The Humanure Handbook. Making good compost from that outhouse is totally doable.

Pasturize any or all of those materials and they should make a fine bulk substrate, cubes aren't very picky.

Search up thermal composting for working on a large scale and putting the humanure to use.

That certainly sounds encouraging, thanks
Years ago, when I grew, the people I learned from made it sound like horse manure was basically essential for tubs so I never got past birthing simple cakes. It didn't occur to me to basically try and grow them on oyster mushroom substrate. It seems I have some reading to do to catch up

It should work. There are stories about people composting used substrate only to see carpets of cubes spawning up on their compost pile

Why do you think the horse manure pellets are crap? I use them for years without issues...

Of course dried unprocessed manure would be better but I don't have car or space to claim it from stables. And of course hobby markets and e-shops sell only the pellets and nothing else.

Sorry, I didn't mean to imply it was bad. I called it crap because it comes out of the back end of an animal I don't have an aversion to manure.
I'm very frugal and it seems counter intuitive to me to spend what it would cost to import a few litres of dried manure, and that would leave one more piece of evidence in some database to be correlated with others, so I'm avoiding mail-order.
Eventually I'm sure I'll be able to get a truck load for the price that two kilos could cost by mail. Its not rare in my region, its just mostly spoken for in advance and I don't want to seem suddenly desperate for horse manure right in the middle of the growing season. People think I'm strange enough already. That's why I was hoping for an alternative to use right away , it could also serve as a backup plan in the future

Would a significant portion, perhaps 35%, of actual garden soil be detrimental in the substrate mix?
The 'products' of my outhouse get layered into a compost pile with lots of plant biomass and the poorest soil on my property, this is composted for a year to yield a product that is probably about 75% improved soil and 25% organics, by volume. I have lots of this. I was thinking I could mix that with an equal volume of home made potting soil and mix that with chopped and soaked grass and brassica straw until a nice consistency.

I'd say that the mycelium would happily grow around the soil, just not as vigorously as if it were all organic matter.

Coco coir blocks are an economical option as well if you want to get on with it without the wait.

Yes I've seen some people not using any manure at all...

I think the main reason for using hore manure is that it is rich in phosphorus. As long as there is enough P in the soil, I think the shrooms will grow happily.

...there is probably one issue with the pellets which is the pasteurization in processing, making my bulk substrate probably over-pasteurized.

The pellets are also super concentrated and hard to "dose properly" I still have very poor yields, my 12 liter horse manure substrate (mixed 1:1 with rye) yields less than a 1.2 liter rye grow box I've been ordering in the past. Something is horribly wrong and I don't know what...

Sorry for going off topic. In short, give it a try