It seems that the MCD500 has both coax and toslink digital inputs, implying that it accepts S/PDIF inputs. FLAC files can be converted into an S/PDIF output with a combination of software (music software capable of playing FLAC) and hardware (a sound card capable of coax or toslink digital outputs). Alternatively, the conversion from FLAC file to a digital output could be accomplished in hardware alone with a Logitech Squeezebox.
The bottom line is that based on the device's specs I can't find any way that you can do it without using a computer to interpret the FLAC files and send them as digital outputs to the unit.
Michael