I figured out a way to secure the backpack in Battroid mode without using the brace!
There are these two pegs on the "shelf" that, in the proper configuration, are supposed to go into two holes on the back. In the proper configuration, these don't actually provide any structural support and are more to guide the backpack into the correct position.
Now, instead of pegging them in, take the two pegs and rest them on the top of the back. You'll also need to pull the intakes as far up as they can go. This actually provides the entire backpack the support and structural integrity required to maintain its position in Battroid mode, even with some light handling and posing. Of course, you'll need to hold it down when moving the arms, and leaning the figure back might cause the thruster section to come unpegged.
However, the only one real downside is that this configuration limits the head articulation as the intake section is slightly higher than it should be. I think it might be too high for some, but it looks okay to me.