There already is: https://www.torservers.net/

I work at Mozilla, and the folks at Torservers.net were extremely helpful in helping us get up to speed quickly. We're hoping to contribute to the public body of knowledge on how to operate servers efficiently, both in terms of effort and cost.

Great, documentation and design contributions from Mozilla would be as valuable as running nodes. That is something they do well.

IANAL, but would this just require someone incorporating or starting an LLC and then paying for the exit nodes in the name of that entity? Would that be sufficient protection?

Also not a lawyer, but you can still be charged criminally in the USA:

"Charging a corporation, however, does not mean that individual directors, officers, employees, or shareholders should not also be charged. Prosecution of a corporation is not a substitute for the prosecution of criminally culpable individuals within or without the corporation"

from http://www.justice.gov/criminal/fraud/documents/reports/1999...

"sole proprietor" LLCs don't have as limited liability as one may hope. you need more people to shift the blame around.