Update #2: I think temperature may have been the problem. Went out and bought a heat mat, taped it to the side of the terrarium. Gave Zacharie a 10-minute honey bath followed by a soak in his normal bowl. He still hasn't passed stool, so I'm still a little worried about potential impaction, but he's now burrowing to more reasonable depths and he did take two wigglers from me. All in all, he's had a big day, so I'm going to let him get settled again. Wish me luck, all!