Since I’d rather not deviate from upstream (nor am I familiar with the package’s codebase enough to modify it), I looked into Help creating a strict snap that uses shared memory and Bashtop Linux resource monitor - #3 by ijohnson. That didn’t seem to change anything, however.
I opened an issue to track this Snap access to shared memory not allowed · Issue #65 · micro-ROS/micro-ROS-Agent · GitHub.