All the examples of tracing that I could find (and there appear to be only one or two that get broadly copied) rely on the .mmcu section and they don't mention --add-trace.

I figured out that I can use the 'trace' kind to get at various registers (e.g., --add-trace ucsr0a=trace@0xc0/0xff) but the other kinds appear undocumented and I can't figure it out by looking at the code.