OK! Progress. I tried manually editing the link.txt file again, replacing the following section at the end of the file:
with the hardcoded path to the dylib:
This is what I had done previously, and it did not work. However, I think that after linking to the homebrew version (as pictured in the screenshot above and suggested by Mio) and then performing this edit, compilation worked and an EXR was output.
For whatever reason, the shortcut reference to the dylib is not working, but replacing it with the literal path to the link works.
Yay for brute force!
Thanks for your help on this, I look forward to testing it.
FYI - the link.txt file to which I am referring is located in: