So, i tried Camtasia and frankly it's not up to the job. It can't keep up with the motion, you just get a jerky version and a quick web search suggests that plenty of others have the same problem. Even if it DID work, what a clumsy way of working - surely we shouldn't have to go out of the Adobe ecosystem to finish a job.
In my case I've made a bunch of cool Edge animations that a Client wants us to edit together and make into a YouTube video - unless there is a realistic answer to the OP's question, I think i'm going to have to rebuild the whole thing in Flash, which seems like a time-consuming and backwards process!
Alternatively, there IS a better way?
thanks!