Claude Barfield has some interesting and insightful things to say about export restrictions here. Here's a brief excerpt where he talks about the prospects for WTO disputes in this area:
While these policies should be countered through negotiations, I would argue (without getting into the dense WTO legalese) that there are two major problems with attacking through the Dispute Settlement System. First, given that one-third of WTO members impose some form of export restrictions (and most of these are developing countries), a wholesale attack through means of alleged violations of existing rules would deepen the already wide divide between the North and the South in the WTO. Further, while such a defense is still over the horizon, claims by developing countries that the export restrictions have an environmental purpose will further complicate and muddle this trade policy area.
Second, as noted above the GATT/WTO rules system does not really adequately cover--and the rules did not foresee--conflicts relating to restrictions on exports. Thus, attempts to expand or twist the definitions of subsidies are based upon weak or dubious interpretations of the SCM agreement. There is already division and conflict on these issues within the WTO. In 2006, the WTO Secretariat, in the Trade Policy Review for China, directly criticized China's use of export tax rebates as an industrial policy tool, arguing that this was an implicit subsidy.