Not really thought about that one to be honest..... Both times I've had to dump fuel, we had a lot of open ocean to do it in! However my opinion, for what it's worth is this :
I don't think I would jettison in any sort of holding pattern if I could avoid it. Certainly not in a terminal area. I would much rather fly in a straight line if at all possible while dumping. If that wasn't possible, then a very large box-shape pattern it would be, giving plenty of time for the trail to dissipate. I guess that dog-legging as you suggest would also be an option. The wind I would imagine would make the trail drift downwind, however the A330 QRH suggests not dumping at below 5000 ft. Therefore I imagine it takes about that for the trail to evaporate. Someone else probably can give you a better answer to that one.
I think the key to the whole thing is to liase with ATC, and ensure that there is no other traffic around you - granted, this may not always be possible. Both times we did it, we a) were in open ocean with no, or very little other traffic. And b) we made the decision early enough so we were able to dump while flying in a straight line to our diversion. Perfect circumstances really! In any event, I would ensure that I had plenty of space to fiddle about in before I jettisoned!
Hope this helps!! Quite though provoking - thanks for that!!
