Hi,
I don't know if this has been done before, I guess it is possible in the sense that one could write a plugin to ABX between different files. However, that would require one file per setting. I guess you could write a plugin which takes a FLAC file, encode it with some settings and then play it back, but writing a good encoder is a really difficult and intensive task. So I think that's why most people ABX on computers. But I have to say I am not expert at all.