Wanted to post a quick update here:
A couple weeks ago, I figured this out with the help of @joyrider3774. Turns out, the Playdate compiler flags were only being applied to my code, and not any of my libraries. I believe the compiler was producing assembly that was incompatible with the Playdate, causing the app to crash as soon as it tried to call Opusfile functions. I adjusted my CMakeLists.txt to add those flags to all of my subprojects, which fixed the problem.
At some point soon, I'll update the playdate-opusfile-test repo with the working CMakeLists.txt, as I want to turn it into a benchmark so I can try to track down some weird performance discrepancies I've been seeing between Rev A and Rev B consoles. I'll make a new thread for that.
Edit: I've now updated the repository, and started a new thread detailing the performance issues.