I know of people who put a backing of sorts on their reel and then tie the braid, but I don't know anyone directly who does this. I have not done this either but the thought did cross my mind, the thing that came to me is, as I always break off a bit of my line before I start fishing (as do most I guess) in a set up like you described, would you not loose more line when your braid shortened and perhaps make it self defeating, unless you want to add new braid with another knot. So maybe it would be better fill the spool with braid and as it lessens out reverse the ends and use a backing, so you get the new end that was at the bottom of your spool now at the top.
I fill my spool quite high with braid as it does not have a memory like mono and so no birds nest.
You will rarely catch a GT in open water, when they take, they dive for cover and try to rub off the lure on some structure, so you want to stop them with a good reel and strong line, so your setup may just work with GT's. With a large threadfin it may be another story as they can run, and as you loose line on your spool the drag pressure will increase putting pressure on the knot, but a good FG knot should work. Having said this the largest GT I ever caught was in open water :).
I have mono on all my large trolling reel, I had braid on one 50lb Tiagra, but took it off as the braid started to snap well below the breaking strength. I test all my line on those large reels with a scale before I go fishing with them. Having said this maybe all of them need their line replaced as I have not used them in years now since the anti-social elements attack in Mumbai.