So, yes, I will be posting updates on Penelope's condition over the next few days, until I know she is in the clear.
As the photo in the previous entry indicates (assuming Blogger posted it), she did eat some food when I got home. Her bowl of dry food appeared untouched, but I bought some canned food on the way home, to try to grease up her innards a bit. (Normally she doesn't get canned food.) She devoured about half a little can of Fancy Feast Tuna, and will hopefully eat some more later tonight.
When I checked her box, there was some poop to be found. Not the three days worth you'd expect, but at least something. So I'm a little less freaked. Which still leaves me plenty freaked.
The current plan: If she doesn't resume normal service overnight--I'll check her box in the morning--I'm calling the vet first thing. Ideally, the vet will say she can wait until Saturday to see her, because I don't want to call in to work with a sick cat. But, whatever it takes...