+1 on Rog's concern. A complete z-lite will not work, however... if you're up for some customization...
My sleeping pad is 1/3 of a z-lite (torso) glued to 2/3 of a GG thinlite pad (legs). When I fold the z-lite part, then roll the thinlite part around it, it creates a cohesive "beam" of pad (about 6.5"x3.5").
This slides nicely into the space where the hydration pocket used to be on my backpack (golite jam). I snipped the hydration pocket and most of the back pad pocket out, leaving a small (4") pocket at the bottom and two thin (1") "straps" to hold the pad in place higher up.
This method creates a stiff central 6-7" wide back support beam, and does not intrude much into the main pack body. Works great. Bet this setup would fit in the REI Flash pad space (might want to measure before you cut).
If you did this, you could send one z-lite back, cut the other one into thirds, and order two BPL torsolite DIAD pads from the store here ($10 a piece), and glue 'em together... then you'd each have a super light pad, comfy on top, and warm on the bottom, and multi-use as use pack "frame"... pretty hard core! :-)