Thanks for this Nater, it's definitively a worth addition. I'm always in favor for more "simulation" or "realism" in games like this.
A few questions/suggestions though.
I don't know how hard it is but have you thought about making it a moodlet (could even re-use the vanilla "hungry" one) that simple says "need to go to the toilet" with variations such as "slight need, somewhat urgent need and urgent need?" It would be a bit more realistic than knowing exactly how much you are ready to go, solves the "need" to add urinating and removes the need to sometimes check a separate window. Seamless integration of a new necessity. Could do the same with diarrhea, but you could just rename it to "food poisoning" to make it more generic. Could start with "queasy", progress to "nauseous" and end up with the "food poisoning" moodlet. It would add a bit to the tension, not knowing what you have for a while.
Second thing, I do agree with Bejasc and being able to interact with bathrooms instead of them being a dead zone in your house would be nice. Maybe possible other bodies of water too, such as rivers. But I think you'd still need sheets or toilet paper when using the bathroom, just less than when doing open defecation. One possibility to make things simpler would simple be to make it so toilets are recharged with TP or ripped sheets, so you don't need to haul it with you everywhere. There should be an option to defecate, in the option or toilet, without TP or anything else, but that would make you sadder and increase chances of getting food poisoning (maybe even with cooked food, if possible).
Using toilets do create a problem though, which is the huge amount of water we use for flushing. I never tried myself but I hear that, in game, if you put a rain collector directly above a sink it will work again, which is nice but for toilets a barrel would be probably good for a few flushes before it runs out. So the suggestion would be to jury-rig a few rain collectors together in an upper floor to create an improvised water tower/water tank. I know that in a few poorer countries, and countries that have trouble with water, they build houses with plastic or ceramic water tanks in the upper floors (the attic, just below the roof tiles) that store a set amount of water and is used through the house. Usually they get refilled with a pump but since you can open them to clean them, I guess in PZ you could just leave them open. This would also make so any sink in the house would be usable, and could also be used in the "showering and personal hygiene" suggestions, which I also support.
For showers, as mentioned previously, I agree it would be nice if there was a need for that too. A moodle for when you are getting dirty, with sweat-inducing activities such as chopping wood, carrying a heavy-load, running long times from zombies or being drenched in zombie guts quickly filling it. The dirtiness would tick slowly otherwise so not doing much running or heavy-work, like just sitting and reading would mean you probably could get away (and would be desirable to save water) without showering for a day, probably for a few, but a week without cleaning yourself would be too much.
Maybe not using TP could increase your "dirtiness" instead of making you sad, so you'd need to shower sooner and both "mods" would fit nicely in a personal hygiene mod. High dirtiness would carry the increased risk of getting food poisoning when eating, even cooked meat, and could you make you a bit sad over time in high levels.
It would be nice to have water as a resource that needs to be divided between thirst and hygiene, carefully rationed. You might end up alive but during dry spells or cold months without any water to shower or using the toilet for days, further complicating the player's life and encouraging less sweaty-work (while also encouraging running only in emergency situations/with abundance of water). It certainly would be less annoying than managing electricity once it goes out (especially since you can't salt food meat and pickling vegetables destroy lids).
Final request: If it wouldn't be too much trouble, please, always keep a non-steam download for the mod. As a GOG owner of PZ, steam workshop only acts as a walled garden for modders beautiful creations.