For me, it's not an issue in the morning to just pee outside out by the inner fences (still dark, and neighbors too far to see (and fenced, anyhow)...but we have female boarders who aren't going to do so, hehe.... The stables are quite a trek from the main house, and even though the shop bathroom is available when we are home, when we aren't home, they basically don't have anywhere to go.
I'm thinking we just wouldn't make it very deep, and we'd use bacteria, sawdust, etc. to treat it. We're pretty far inland, so that's a good thing. I just don't want anything that would contaminate the well. One idea was to even make a small concrete vault about 4' down, for it, so that may pan out. For the walls, just 2x4's and plywood. (doesn't really usually get too cold here, so insulation isn't really a need.)
When time to empty, I'd probably just have a sewage company come out and pump it.
We're in an unincorporated area, but I'll still check any local laws regarding it. (though really, nobody would stop me from it, or even see it being done).
I'd do it up nice. Actual toilet seat, power (so motion light, exhaust fan, cooling fan, automatic fragrance, even outlet with USB ports for phones, etc.).
I'd use the tinted corrugated plastic roofing for the ceiling (to let light, but not prying eyes in)...and frosted glass for windows.
I'm thinking I could even have a tube loaded with sawdust, and treatment materials, that could use gravity to deliver a dosed amount with a "flush" like trigger.
I could even splice off an existing water line in the barn, and run it to a small sink in the outhouse (drain water would simply go out to ground) for washing up.
Even put up a little medicine cabinet with mirror door, etc. (with some OTC meds).