Your body IS being constantly pressurized by the atmosphere, and your various sphincters are used to that. Presuming the suit doesn't pressurize your body beyond what it's used to (at which point breathing would be difficult), there should be no unexpected anal excretions due to the suit.
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But the pressure from the atmosphere applies to both sides of the sphincter, resulting in zero net pressure. Unless the suit actually does press against the outside of the sphincter like it does the rest of the body, I think OP's concern about the suit squeezing you like a tube of toothpaste is valid.
Maybe the suit only applies a few PSI instead of the full 14.7, which it seems like one's sphincters would be able to withstand.
I think it's like a third of an atmosphere or something. Enough to comfortably achieve the same partial pressure of oxygen as normal Earth air, by providing it pure.
Probably not? I think the suit would have to hold all of you together to work otherwise you would risk parts of you following the waste.
I'm also pretty sure the vacuum doesn't even top the list of things you need a space suit to protect you from. Eva suits do a lot of work.
In terms of engineering, it does. Micro meteorite protection and heat management can both be provided by normal garments. UV protection is obviously easy enough too. Breathing gas is a bit less convenient, but still not as tricky as making a suit that's both rigid enough to reliably hold several PSI in and flexible enough to comfortably work in. That's why the elastic suits are being researched like they are.
... That's actually a good point. I'm guessing since the digestive tract is flexible and isn't held open to the outside all the time, it wouldn't cause problems with things deep inside. I also think it's inevitable that if you did shit yourself in it, suction would kick in at some point and make it all a bit more dramatic. And then it would boil-freeze off into space, and be icy cold. That might still be better than pooping a sealed space suit, though.