I use my 2x converter with my 70-200mm F2.8 VR quite often, as it give me the reach if I need it without degrading the quality too much. I have a 400mm F2.8 which weighs 6kg and use that with 1.4 and 2x converters quite often also with great results.
As you state in your original post it's a once in a lifetime chance to get some cracking photos. If the Canon 70-300mm IS is as good as the Nikon version with VR then, I don't think you will have any problems, as long as the conditions are quite bright for most of the times.
Unfortunately the conditions will be only bright for roughly 70% of the times....the rest dull, overcast, rainy and bloody windy to boot.
Altho i'm told one can take tablets for wind....but not the rain !!!