Well, good luck to you on that - I don't know of anyone who has successfully kept a plug in a head for any length of time using JB Weld - but I also don't know anyone who has tried it and had it fail and admitted it, either. Please post a followup if it works for 50k miles or if it fails sooner than that. At the worst, if it fails it will not have damaged the hole further.
Unfortunately with the Zetecs there's really no possible way to repair a plug thread without pulling the head. The hole is too deep in the head.
I've see a lot of criticism over the years on using helicoils for plug repair but the one in my Zetec's head has been there for around 30k miles now. And most critics of them seem to be Timesert distributors so they are biased anyway. I still maintain the effectiveness of a heli-coil for plug repair is directly affected by the experience and skill of the mechanic putting it in, which might account for why so many people criticize them - because it's easy to criticize something you don't have the skill to properly do.