In both v0.65 and the latest CVS dump from http://www.aep-emu.de/cvscompile/dosbox.htm, the CON device is not handled properly:
[begin rough transcription]
COPY CON FOO.BAT
File not found
[end rough transcription]
Note the double space between the words "File" and "not"; this may be a problem with the builtins instead.