Well Folks, after a solid 15 years of looking all over the place, and never quite certain about finding, I finally found some legit P. cinctulus growing from straw (hay?) bales set up as erosion control.
It was a magnificent feeling, seeing these things. What was not a magnificent feeling, was coming back about a week later after I thought conditions were perfect, ready to feast my eyes on pristine, bountiful Panaeolus cinctulus, a sight I'd been seeking for approximately half of my lifespan, only to see a solid cluster of these babes all dried out and past (no pic of that one). I came back after every rain after that, but they never showed up.
Oh, well, I'll take what I can get! I did get a spore print at least, so maybe some day when the time is right, I'll summon them from the microcosm. It may be a bit late now, but I think I'll grab a piece of that hay bale and stack another couple bales over it, just in case they decide to pop next year.