I wouldn't think there would be any benefit to installing the center support prior to hardening. On my last couple of bare receivers, which were fully heat treated, I had no warpage, but I was using 1.5mm flats. 1.0mm flats would definitely be more susceptible, as I was able to warp one years ago with a bad spot heat-treat; however, if your receiver is heated consistently throughout and the dunk is vertical(and quick), you will probably have good resultes. A small amount of warpage is ok, too. Just leave a little bit of excess material to file on the upper rails. Once you install the center support and trunnions, a small warp should straighten itself out, as I found out with the above-mentioned receiver.
One other thing to consider: if your center support sleeve is a hardened steel, and is not the same material as your flat, it may not respond to a water/brine dunk in a positive way. It could soften, which might not be the worst thing in the world, or it could crack, which would definitely be bad.
Either way, good luck!