TiBike
OBS Chief
They do this on the Surfliner. Business class sometimes extends part way into an adjacent coach. It's the same snackbox/wine/2x2 seating as in the dedicated business class car. I think it's fine -- much better odds of your seatmate and neighbors being agreeable travelling companions.
You could also dynamically auto-adjust available business class seats by rows instead of entire cars based on demand. That way no more than a row or two of buffer space would be "wasted" on business class. This is very similar to how regional length business class is handled on some European airlines. But, it also has the distinction of being very efficient and I don't think that's something Amtrak appreciates.