In regard to the use of a lever to turn on only one parking brake to lock 
out a wheel for extra traction...

JC Whipme used to carry such a device for Bugs years back...and I've seen 
them advertised in VWT&A, etc....I had one installed in my 74 Thing and it 
worked well when the odd time came along that the thing got stuck.

The problem with the bus it that the mounting arangement for the parking 
brake is much different than that of a beetle. At best you would have the 
levers blocking the center of the floor (they were about 12" long).

Ric