Wisconsin and International Labor Law

I have not seen too much discussion of the international law features of Gov. Walker's proposal to end collective bargaining rights for state workers in Wisconsin.  The right to collective bargaining is one of the so-called "core labor rights" that the U.S. uses for GSP conditionality, and the 2002 TPA required labor rights to be a negotiating objective in U.S. PTAs.  Wisconsin's statute, if passed, will make this a bit ticklish.  I wonder whether the right to collective bargaining is now viewed as customary international law.  I take it that there is no treaty that the U.S. has signed that contains that right directly.  Article 6 of the US-Jordan FTA , for example, however, requires that each party "strive" to ensure that its laws incorporate freedom of association and the right to collective bargaining.  I suppose King Abdullah might be a bit preoccupied at the moment, but it is tantalizing.  (I assume but would like confirmation that Wisconsin is exempted from the NLRA.)