What's causing the sewage smell in my laundry room?
Well, I'm not sure which of the following actions fixed it, but the smell seems to be dissipating after letting the room sit overnight afterwards:


I cut the vent pipe off in the attic right above where it came out of the ceiling. Then I drained any stagnant water due to the "U-bend" in it.
I snaked down from the roof to the cut location, with no obstructions found.
(Due to more luck than planning, the run was within a few inches of the 25' length of my auger!)
I snaked from the vent cut location in the attic down as far as my auger would reach, and don't think there was anything there, since it came back clean again.
I rerouted the vent pipe in the attic so that any horizontal runs had at least a 1/4" per foot slope upwards so any sewer gasses wouldn't be trapped.
I snaked from the laundry room drain pipe as far as I could reach, once again no obstructions detected.
I borescoped the drain pipe and saw that the trap was full of water as it should be. I added a gallon or so of water anyway, then maybe a cup of vinegar to help with any smell that might be in the pipe itself.


After all that, the smell dissipated overnight. Unfortunately, I didn't find any obstructions, and discovered the trap was full of water. The only thing I really "fixed" was the vent pipe slope, but that's been in that configuration for 7+ years without any trouble, so I still don't know what actually worked!