From what we saw in the whitebox-clip, there's plenty of open hangar space left after the Rover is parked within the Corsair. Can't there be a few SCUs left over although the rover is on board?

We want this ship to have similar trade-offs as the Constellation in terms of 'cargo space versus vehicle storage', so currently there are no plans to allow this. However, there will always be space to put cargo items outside of the grids on all ships providing you can get them in, though they won't be securely locked down.