Ummm we don't call it a "backpack" now, we call it a dumpy. But if the GSC stand works then great. If not, an Action Base will in fact work. They come with a billion varieties of connectors, including the standard 3mm pegs that ATK Girls use, a 5mm, etc.. You can caliper that shit to get a match or just poke at it until it fits. Of course you can also sand down a peg or thicken one up with sprue goo. Only turbopleb snapshitter redditors with no modeling skills are beholden to off-the-rack solutions. As long as her gear stays up on her back, you can hold the whole thing up with an AB. They hold my Sinanju MG in a flying kick pose with its bazooka sticking out and that's been in place for a year now. I recommend taking off her big blade things and posing her how you want her, mounting her solidly to whatever stand, then fitting the blades on after.
I will say I love the ATK kits and they are a 10/10 value for the money but they do have a little more fiddliness and some issues next to a Japanese kit. Stiff joints, loose joints. But nothing that's not perfectible with a little work. I had problems with the spider chick's (human) legs wanting to fall right off their pegs for instance. Easy to fix but it did require fixing.
The two beetle girls and the White Tiger are the only ones I don't have, I think, although I have yet to build up the scorpion chick.