I spoke too soon, the snail inside was still living and only a temporary guest. I have no evidence that he's eaten for over three weeks. Given this, I just got a little bit radical and evicted him from his PVC den. He climbed onto a rock and was very reactive in terms of behavior and color change. He hasn't grown at all that I can tell, and to some degree he looks odd: his eye stalks seem somewhat disproportionately large and sometimes has a pale spot near the tip of his mantle. Also, his mantle appears to have a somewhat 'clumpy' appearance. All eight arms are fully accounted for to their tips.
I tried frozen krill again. He grabbed onto it and brought it to his mouth each time I offered it, but each time he ultimately rejected it. I also took some snails and brushed their feet against his suckers, but he wasn't interested.
Finally I grabbed a female fiddler and tried to work it between his arms. This interested him a little bit, but he generally moved away from it. I decided to get aggressive and started slowly chasing him around the tank with it. He did try to engulf it a few times but ultimately gave up. Eventually he made his way back into his old PVC den. I put the crab in with him, then blocked off most of the entrance with a Lego, making sure there's enough gap for water flow.
These actions are pretty aggressive on my part and I acknowledge they may be stressful on the little guy. On the other hand I don't think he's eaten at all and I think some degree of intervention is necessary. Luckily he didn't seem too stressed by my attempts to feed him: he didn't at any time show 'terror' colors, ink or jet. When he did withdraw while I had the fiddler it was never very quick. I hope now that he's stuck with that little fiddler for a while he'll go ahead and take it down.