I imagine it might be possible with some iso hacking. People have tried it with GameShark, though I'm not sure how successful they were. It would certainly rock, 'cause Dark Serge had a really awesome character design IMO.
EDIT: Actually, we certainly have the capability of changing Serge's portrait for Dark Serge's at this point in time. Basically, you'd need to find out where Serge's portrait is in the iso via hexadecimal offsets, then simply do a cut & paste of Dark Serge's face file (It's of the .TIM file type) overtop of Serge's in a hex editor. But changing the face won't do ya any good till we figure out how to rip the models.