You're right, Mintapple's art reduces surprisingly well!
However, that's not palette-reduced, so a drop in quality would occur from that example.
If the CE team is willing to consider taking non-pixel art and converting them into CT portraits, I'll throw tushantin into the mix. His art style is way different than Darkken's, but he's seriously improved as of late. These are some of the ones he's done for AE:HU.
Still, I just love Darkken's portraits to death. They have an incredible charm. I'd say it's best to leave things as-is if Darkken can't be found (unless of course someone can emulate Darrken's style). From the player's perspective, it's believable that the other characters wouldn't have changed as much in 5 years as Marle, Crono, and Lucca have.