Does anyone know what you have to do to unlock *all* of the 150cc bikes
in Tournament mode?

For 50cc and 100cc, they were all unlocked by the time I took 1st place
in each of the four races in the respective class. For 150cc, though,
about half of them are still locked:

Rumbler 150cc
Little Dipper 150cc
Zipper 150cc (locked)
White Lightning 150cc
Flow 150cc
Gearhead 150cc (locked)
Puddle Jumper 150cc (locked)
Blur 150cc

Granted, worrying about this is borderline OCD, but I want that Gearhead.