After making a few needed tweaks to get libbpg to compile on Mac OS X, I used the compiled bpgenc binary to convert a test PNG to BPG format. I also converted the PNG to a JPEG for comparison purposes. You can see the results here:
http://justinmayer.com/bpg-test/
Size of PNG before conversion: 186K
Size after conversion to JPEG: 52K
Size after conversion to BPG: 9K
I took the liberty of submitting a Homebrew formula, so hopefully this will soon be a quick "brew install libbpg" away. (^_^)