I can play with the bags this weekend and see about moving them around (forward/back/up/down)... but so far, it seems that the top corner or the Overland30 bags will poke up over the seat a bit. I will check that this weekend. That said, if the bags are not stuffed full right to the top, they will compress down a bit.
As for the buckle on the seat question. While I completely understand wanting the seat clear of the buckle. I would question why you want to try to move the weight so far back on the bike. One of the things I really like about the Overlander30 is that it allows the weight to be lower and more central than many other systems. Is it possible to just have the buckle more in the middle of the seat and on your highway jaunts you can move "over" and behind the buckle?