Henry, you're right about the dimpling; I hadn't taken that into consideration. I think a layer of masonite would be my answer to that problem, but that doesn't end all the concerns.
I wasn't thinking of keeping the marbles in place at all, except to contain them under the disc with a lip at the outer edge, allowing them to roll completely free within. Also probably not a good idea since flex in the disc & stage would allow them to "gang up" unless you almost totally filled the space like you mention, Henry.
And of course there's setup time, if the disc would need to be placed for Act II & removed for Act III; I can just imagine the distress of 10,000 marbles accidentally unleashed in a dark theater amidst the rush to move props.
I did awake today with another concern of my own, too; breakage. With a good coverage it probably wouldn't happen, but I know marbles weren't made for this use and aren't consistent in their strength. All it would take is one crumbling to break others as they rolled across the shards while under pressure.
Overall, I think I'm back to carrying the cast with casters, too, Henry; lots less room for disaster.