I'm gonna have to go with a No on this one. Your 100oz Platypus is 18.75" long, and on a Terra 55 I measure the total usable depth of the hydration sleeve at about 17" to the top of the framesheet. Width of the sleeve is about 7" vs 7.75" on the bladder.

I actually use the 100oz bladder from my military issue camelback in mine. I only fill to 2L to save space and weight but it fits fine.