I don't think there's anything in Taft-Hartley that would prohibit that. Right to work laws are state laws in force in 22 states. Each of those state laws are different.
Beyond what the state law says, whether the employer approves the posting would be an important factor since it's their property. There could also be some personell privacy issues. The answer would most likely depend on where you are located. You may be able to get a more specific answer from the National Right to Work organization
http://nrtw.org/know-your-rights
Answered Dec 13, 2010
Edited Dec 13, 2010