Implausible, but not improbable. 
I mean, Blue Superia Cruiser and Queen II Cup are both clearly a fake, but original game coders could have done it without problems (and it would have been really surprising to unlock).
There could have been only two problematic tracks, due to Mode7 limitations: Fire Field: Water Fusion and Bianca City: The End Of The World, with that "double fire/waterfall" and that "bigger ship leaving the sky with all the racers inside". Sorry, but as I'm a graphic artist and a beginner programmer, I can't resist to comment about hardware capabilities of the console... 
I also remember someone saying it was possible to drive not only the blue car, but the yellow and green ones, like Yazzo said. 
Stil, I really can't understand why that blue car is only one, while other yellow or green drone cars have been copypasted along the circuit. :S
EDIT: oh, I forgot to say... Actually, searching for Blue Superia Cruiser on Google Images brings out pictures of bicycles. XD