Well I think it depends on how much its made necessary and also how relevant you can make the mechanics.

As far it being necessary -- people don't really need to wash or bathe as much as they do in modern times. They certainly didn't always do it as commonly all throughout history. So, its a matter there being certain circumstances in the game which would either be a reason for you to bathe or wash.
Then as far as relevant mechanics goes, your reference to other forms of realism I think hits this point. For me, its not so much that these things are so far out to include in a game; you find them all in a game like The Sims, for instance. Its just that there aren't any relevant mechanics that could serve them well in a typical RPG -- or that I can think of in UrW -- because there's not much meaningful gameplay or consequences associated with doing them or not doing them. What's the game going to do? Tell you "you defecated in your pants like a four year old, this embarrasses you greatly"? As far as bathing and washing goes, you would then need particular reasons to do these things.
But I can think of a way of implementing them both where both the need to do these things would be rare enough and relative to the circumstances and there would be benefits of doing them. Even just for roleplay in some circumstances; because lets say you want to marry a bride and can't do so while you're dirty, so you take a bath. And I can imagine, as I mentioned in another thread here, villagers themselves bathing and washing in rivers as part of their routines.
In any case, I'm sorry if this diverts from the topic of your thread a bit; I just thought it might be relevant to suggest it because if Sami and Erkka ever think of adding a wetness property to clothing, maybe there could be thought to other states and conditions as well.