Ah. Sorry. Do give it its OWN directory for the build.
Yes, that does work. But I was worried that Arduino will delete all the files after a verify or upload function.
What I have found is that Arduino does delete all the files in the build directory, but only on exit, so if I keep the IDE open, the output files can be examined. In particular, it is possible to apply objdump -S against the .elf file. And the output from that is exactly what I was looking for.
Thanks for all the help.