What gets explicitly included in the main chain via transactions are the (unique deterministic) beacon checkpoints, at most one per 120 periods. The second-most recent beacon checkpoint is the “global hash” that Casper FFG can finalise over.
Ah, I see. Missed that part. That actually aligns well with the general approach I just finished writing up today 
Off-chain intermediate blocks
I’m not convinced that a separate random number chain is worth the complexity, but aside from that the paths seem similar.