I've been advised that it is best to do dry hop after fermentation is done. I usually do that, drop the hop bad in for 3 or 4 days and then keg or bottle. I time it that way so I don't have to open the bucket to get the bag out and introduce more O2. Not sure if it works or not. I am thinking of doing the magnet idea so I don't have to open the bucket lid at all.
With that being said, I don't do a diacetyl rest per se. My closet does what it does temp wise and has always been between 65 and 68 or so. I am trying to be more patient and leave it in the fermenting bucket for an extra week once I see the airlock slow down. I take my greavity readings every couple of days and when I notice little to no change, I dump the dry hops in, wait 3 or 4 days and then package.
Take that for what it is worth, as I am still new and my process is still being tweaked to the situation. RR