Here's the base for the staging yard -homasote screwed to a plywood base.
The plywood under the homasote is a bit thinner than what I'd use for subroadbed and consequently useless for that purpose. But as a base for homasote, it works fine.
The resulting sandwich is thicker than my usual subroadbed/cork roadbed, so I'll have to ramp the staging tracks up slightly. But I think the resulting ease of staging track installation (instead of using more cork or - worse - attaching the staging tracks directly to the plywood) will more than make up for the ramping.
I'll paint the homasote to seal it, and then it's off to start benchwork for the Essex to East Haddam portion of the Valley Line. I'm going to need much more than a spare hour for that. But I'll make it eventually - one small bite at a time.