Thanks for the helpful responses, everyone! It's comforting to hear that Zoe's not the only one going through this. Within an hour of my writing, Zoe had a nice poop at Nell's house, and I don't think it was any kind of food/drink in particular that got her there... she just decided it was time to go.
I like the idea of coaching her to "push it out" more regularly as a way to feel better (even though she says she feels fine), as there's obviously poop there after a couple days. I'm also going to try adding wheat bran to her oatmeal (she loves oatmeal) and will have miralax on hand as well. We've on occasion added prune juice to her milk and she didn't seem to mind (although lately we've cut back on milk altogether).
Fun stuff! Have a great day, everyone!