Honestly? You are seeking perfection in an imperfect world! If you strip your car to the bare steel, you will find any number of little bits of surface corrosion hidden away and probably some not-so-little ones too!
It's hard enough to find a wing these days that ISN'T full of more holes than your ma's colander! This little bit of surface rust isn't going to bother it, ask yourself how many Dolomite wings you've seen that are rotted in this area and the answer will be a very low figure, probably zero! My inclination would be to clean it up as well as possible, treat it with something to stop oxygen getting to it (like lots of PAINT, it can't oxidize if oxygen can't reach it) and fit it and forget it. It's in an area where chips can't reach it, the rest of the wing will rot away long before that bit does.
If you really MUST remove the flange to treat it, it IS doable as others have stated, it's just a matter of finding the spotwelds that secure it, there are only 3 usually, cautiously drilling them out and then carefully opening up the turned over section a bit to release the closing panel. I once bought an otherwise very good wing that had been removed in this fashion and managed to reconstitute it with a closing panel from another (rotten) wing to make one good one. I got decent money for it too!
I can see your point, but it seems a shame to go to all that work and expense and leave rust in the seem.
To my cost I have found quick fixes just don't work with holes and rust. In theory stopping the oxygen reaching it should work, in practice if the reaction continue the expansion of the rust will break through your protective coating.
The only thing I have ever found that works is to grind and cut the rust out. Otherwise you will be seeing it again.