Thanks for the suggestions. I ran this past some club members at the meeting last night and got some other valuable ideas, as well.
The degree of curvature is more than I thought, like 1/4".
I did try the sand paper on the cast iron top idea, but that's going to take quite a while. By the way I suggest covering the top with craft paper otherwise the cast iron will be scratched by loose grit. And yes, there's a reason I know.
Another suggestion was to bring the blank back to Make It Here and use the ShopBot to flatten the bottom. I wouldn't be able to do that before tomorrow.
So based on yet other suggestions this is what I'm going to try. This is just a mock up, I will use a screw chuck, but the vacuum was on the lathe.
Sand paper on a board with double sided tape. The board mounts to a face plate and spindle adapter and is restrained from rotating by the ways.
I think this was my favorite quote:
Some people call this space a studio. There is artistry...but it's a shop. It's noisy. It's physical. Machines play a big role. It's not a factory where parts move from station to station manned by operators. Parts do move from station to station, but they are accompanied by crafts men and women whose hands, eyes, and hearts are engaged in the steps and invested in the outcome.
That is a Stanley #52 shooting board, with the Veritas Shooting Plane (in place of a Stanley #51 plane).
You may wish to read this: http://www.inthewoodshop.com/ShopMadeTools/ShootingforPerfection.html
Regards from Perth