Why does it have to be a "back" pack?
Before now, I've taken backpacks for people who've hurt themselves and needed help. Since I've been carrying a pack myself, I've put their pack on my front. I can report that the result is well-balanced, fits fairly well if it doesn't have a structured back, and causes no problems at all. The only issue for carrying a regular backpack that way is that it can be hard to see over the top of it, since it's designed to go behind your head. Any race with a reason to need their back free could easily shape a pack to mount on their front, with no other ill effects.
You could also consider that it doesn't need to be a single large pack. Humans use those because it's practical. There's no reason a "standard" pack layout for another race couldn't be a harness with both front and back packs, containing the same overall volume but arranged slightly differently. Other possible pack layouts for a flying race might also include pouches strapped to the legs (you don't need those when you're flying), or even simply a large underslung bag dangling on the end of a rope.
Edit based on ChrisH's comment: Much of this mirrors how paratroopers carry equipment, because a paratrooper's back is occupied with their main chute. Their reserve chute goes on their front; various equipment is secured elsewhere on their body, and their main pack is, as I said above, hung on the end of a strap.
Forcing a non-human race to use a "back" pack simply because the rules were originally written for human-bodyplan PCs seems like an extreme rules-lawyer stance which most players would not accept.