Is it feasible to 'harvest' the clathrates from the permafrost before they evaporate?
No, it's not feasible. They are too dispersed, and any operation to harvest them would be too costly and inefficient, and would amount to vandalism on a gigantic scale. We just have to hope that the vast majority of tundra clathrates will remain un-melted, as will probably be the case. The last time they were melted on a massive scale may well have been the Permo-Triassic extinction event of 250 million years ago.The Arctic tundra is a vast natural wilderness full of valuable wildlife. Any ill-judged operation to remove clathrates would not only be grossly expensive and inefficient, but would have both taxpayers and environmentalists up in arms! 

Assuming theories of the Permo-Triassic event are correct, it took the long term eruptions of the Siberian Traps super-volcano to release almost unimaginable quantities of CO2 into the air to trigger the melting of clathrates. Eruptions of super-volcanoes like the Siberian Traps 250 million years ago and the Deccan Traps 66 million years ago make modern volcanic eruptions, even Krakatoa (1883) or Tambora (1815), pale into insignificance.