Reverse bracing is not essential, and will only have a small effect. If you just want to protect the back, don't bother.
You can process the fibers yourself, but why spend all that time pounding, shredding, washing etc... when you can have it nearly for free? Moreover, the agave juice can make your skin hypersensitive to UV. I did it once (won't do it again). I pounded leaves in the sun, and had blisters all over my arms that evening, from tiny drops of juice that spat on my arms...
You can also use sisal binding cordage (usually two plies), you'll have a bit more work untwisting it and getting long enough fibers to work with.