I plan too. I took apart my old (non sj) cam and my SJ yesterday to eye up how easy/hard it would be. It appears straightforward, clipping the old mic off and just soldering 2 wires. The part for the SJ that the video does not do and has me thinking is how to then create a hole in the separate waterproof case and keep it at least "weatherproof".
Only issue I've come across so far is finding the right jack, though I've only looked briefly on eBay. A low profile panel mount like this is needed
http://www.ebay.co.uk/itm/1-8-3-5mm-...item1e8d3ce9f7 but I'm dammed if I'm paying £13 for one from the US, all the UK jacks of that type seem to be 1/4" (6.5mm) not the 1/8" needed. The chunkier things for sale with the plastic bodies on them I don't think would be an easy fit. They must be sold somewhere just it's low down on my to-do list.