I've included a patch to audio_coreaudio.c that allows coreaudio device specification on the command line, e.g.:

[dbook: /tmp]$ esd -d "Built-in Input"
- using device Built-in Input
using device Soundflower (2ch) for output:
        with sample rate 44100.000000, 2 channels and 32-bit sample
using device Built-in Input for input:
        with sample rate 44100.000000, 2 channels and 32-bit sample

A complication is that a specified device may not have both input and output capabilities, in which case the driver resorts to the default input or output device.  In the above example, "Built-in Input" does not have any output channels, so the driver selected "Soundflower (2ch)," which is the default output device on my system, for output.

Specifiying a coreaudio device through a configuration file or environment variable is still problematic since device names may have spaces and spaces are treated as separators in parsing these.