There are GUI FLAC players for OSX. However, none of them that I know use Core Audio, so they can't play back FLAC files greater than 16/44.1 over hardware that supports the higher resolution files. There isn't even a command line player that I know of. There is a command line wave(/aiff?) player bundled in the Core Audio dev tools called "aplayer." It's written in C++ and can play back up to whatever resolution your Core Audio hardware supports.
I have given up trying to add FLAC support to that but it would be the perfect starting point for someone who knows what they're doing. The GUI is the relatively easy part once the command line version exists.