If I put a .o or .a file in my sketch directory, will the IDE include it in the link step?
This would be nice for including "binary blobs" that had been converted to elf files by objconv...
I don't know the answer but I have an idea that may work... Place the binary file in a library directory. I have a vague recollection that the builder will include it when linking.
The .a file may not work. Libraries are "weakly" linked. Without a reference to whatever you want to include the linker won't include it. Of course, if you do reference something inside the library the linker will try to include it.
The library workaround will definitely work:
https://arduino.github.io/arduino-cli/latest/library-specification/#precompiled-binaries