There have been some brief references in the jurisprudence recently as to whether GATT Article XX provides an exception to agreements others than the GATT (see here and, to a lesser extent, here), but no substantive findings on the issue. The China - Exportation of Raw Materials case (DS394,395,398), for which the U.S., EC and Mexico just requested panels, may give us some answers, although there is a good chance we will all be disappointed as a panel once again avoids the issue.
As set out in the panel requests, the measures at issue are: (1) "quantitative restrictions such as quotas"; (2) "export duty rates, 'temporary' export duty rates, and/or 'special' export duty rates"; and (3) "other restraints on the exportation of the materials, administ[ration of] measures in a manner that is not uniform, impartial, and reasonable, ... excessive fees and formalities on exportation, and [failure to] publish certain measures pertaining to requirements, restrictions, or prohibitions on exports."
The claims are under various provisions of the GATT, and also China's Accession Protocol and the Working Party Report (incorporated into the Accession Protocol).
Press reports suggest that China may invoke the GATT Article XX environmental exceptions as a defense:
Assuming China invokes this provision in relation to the claims under the Accession Protocol/Working Party Report, the panel will be faced with the question of whether Article XX provides an exception to these instruments.
If China's Article XX defense is strong and it meets all the Article XX(g) and XX chapeau criteria, the panel will be forced to decide whether Article XX actually offers an exception in this situation. Unfortunately, there is the possibility the panel will simply assume for the sake of argument that Article XX can apply here, reject the Article XX defense on substantive grounds, and then find it unnecessary to decide whether Article XX applies outside the context of the GATT (which is essentially what happened in the China - Publications case). This would be disappointing for those of us who would like to get some clarity on this issue, but would not be too surprising. It all depends on how good China's environmental defense is, which is something I don't have a sense of from reading the press reports on the case.