Actually, I kind of started from the same place. My first thought was a bunch of the big ball bearings that are mounted in positions like on table saws outfeed supports. And a possible note of interest is that my outfeed support uses PVC pipe for a roller and electrical conduit for a shaft of the outfeed roller. Excuse the digression.
The problem that I have with the marbles is that you will have a lot of weight on the wood as a 200 pound guy just stands in one spot with a couple of marbles under that. With so little square inches of actual supported area, the pounds per square inch will quickly overcome the ability of the wood to support without dimpling. I think that will end up being a mess quickly and movement of the moveable disk will stop. I know that the structural plywood is not supposed to have holidays and voids but it does. This will make the problem worse and there is them just a ply or 2 to support the weight. A metal sheet under the disk would cure the problem--but at what cost?
The next problem would be keeping the marbles in place. You would have to have no gap bigger than a marble. Having a circular "track" for the casters to run on prevents this problem. I thought that a gridwork of 2x4s with dados cut to achieve an "egg crate" kind of support would not be a bad way to support the middle of the disk--and you could buy the "stud length" 2x4s that are a lot cheaper than full 8 foot lengths. I would run the gridwork on about 16" centers, but it's the grid that does the work in keeping the disk flat so try for tight joints. A lot of drywall screws and some construction adhesive will help too.