I've thought about that too. But my idea was to have a device kind of like the space rings in RCT, but instead it only rotates in one plane, so when it was desired, the train would go through a barrel roll, and it would switch from floorless to inverted (or vice versa) by sliding just like the space rings. There would also be a locking system keeping it from switching when it wasn't wanted, like if the floorless portion of the ride was to have a barrel roll, but not have it switch from flooress to inverted. In my mind it would work similar to the 4D coasters where there is a second set of wheels that causes the seats to rotate, that way it wouldn't rotate when it wasn't supposed to.
I also like the idea of the MCBR. But how about this, you have a Pre-lift (or launch, whatever it may be) Beer Run, and a MC(T)BR. Mid Course Taco Bell Run. Or you're drunken fast food joint of choice, Taco Bell just fits the acronym better.