Based on a couple of these responses it seems that there may be some logical cause for this problem. The lobster model was accidentally modeled and rigged backward, with its tail in the positive Z direction and head in the negative Z. In order to correct this, I rotated both the lobster's skeleton and geometry, noting that all the relationships still worked fine afterward, and set to creating a cycle. The model bone itself was also reversed in the process. I kept all of my initial changes but re-reversed the model bone, saved and re-opened the project with the action.
The stride length marker works!
Thanks for all your helpful, if slightly bemused, responses! I'll keep an eye on the model bone in the future.