@claudio-ficara Thanks for the suggestions =) I tried my demo with these alterations and unfortunately the results are not much better. The FlxSpriteGroup does not seem to respond well to setGraphicSize() — for me, it disappears from the screen entirely (or fails to draw?).
I thought the center offset was a good idea, but using that method, the green and white FlxSprites are now centered on the lower-right corner of the red boxes. And the FlxText and FlxButtons now float away into negative space for the smaller sizes. One plus is that green and white FlxSprites do align better with each other though (i.e. the border is consistent for all sizes), so there might be something there if you had a group containing only FlxSprites.
Eventually, I gave in and decided to create a new FlxSpriteGroup and resize/re-position the group members individually, instead of trying to get HaxeFlixel to adjust the group as a whole. This works for me, since I don't have too many members in the group, so I guess this method solves my issue for now.