Yeah I think i just read about this on the forum. Kind of strange to deal with magnets TBH lol but I guess if its food grade and has been tested i could try testing it out. I do have that exact problem though, not enough head space within a carboy for 5GAL of wort and anything else.
I remember the 2nd or 3rd day there was vigorous effervescence. Anything within the headspace would have had wort juices circulating through lol. I was thinking about getting another fermenter. Or at least try and figure out a way to get hops in without exposing to oxygen. I really like glass, there is less friction for particles to grab onto and they all fall to the bottom for a clear beer which is my preference.
I have 2 5 gallon glass carboys that I am just using to store Starsan. I would be more than happy to get rid of one if you are interested. Two things though, the openings are only about big enough to put a stopper and an airlock thru and once filled they are incredibly heavy to move around. I have read some pretty big horror stories about dropping them and shattering glass cutting people. I don't use them for anything but storage for that exact reason. If you are going to dry hop beer, you would end up just pouring the pellets thru the opening, thus exposing to O2, which is what you are worrying about. So, not sure this is a good fix for you.
As far as opening the lid to drop a bag of hops in it. I do it all the time with no perceived ill effect. I get the small bag washed and sanitized, grab a few marbles and sanitize them as well. Put the marbles and hops in the bag, pull the draw string and bring it to the bucket. Open the lid only partially, easily slide the bag in, seal the lid back up again with the draw string hanging out so the lid holds the draw string and the bag above the bottom of the bucket. A quick open and shut of the lid should not be a problem in my mind. And I have done mostly hoppy IPA's and Pale Ale's like this. Given them to friends and have gotten nothing but good reviews. And these are cops I give them to, so anytime they get a chance to complain or burn me, they do. LOL
As far as the magnet idea, I have seen lots of guys use it when they are dry hopping in a keg. They use the magnet to hold the bag above the beer, when ready they just pull the magnet off the side of the keg and off the hops go. They then let the hops sit and carb and serve from that keg. To me, any transfer of the finished beer to another vessel means more chance for oxygen exposure, so racking to a secondary bucket might not be the best way to go. In my mind, get your dry hops ready, crack the lid of the bucket just enough to get the bag in, let it go and seal the bucket, all done. LOL
Now, with all this being said, I am in no way a pro and only have been doing this for a couple of years. I average about a batch a month or so, so please take that into consideration when reading anything I post. There are folks here that are way more experienced than I am. Keep at it, take good notes and you will find you groove and what works for you. Rock On!!!!!!!