This effectively is what I did when renovating my house. The cavity walls already had polystyrene boards (under the pre-2002 Building Regulations) and I recently had the beads blown in as extra insulation. I think that there may be advantages to this approach. First, the current polyiso boards have a better insulation value than the beads so you are getting overall better insulation than with the beads alone. Second, the beads are pumped in under pressure pushing any boards that have moved away from the inner leaf back against it. Second, although in theory, and hopefully in practice, water should not transmit across the beads, if it does the foil backed polyiso will act as a water barrier, protecting the inner leaf.
On the question of whether the cavity is completely filled by the beads, the beads are pumped in under pressure and flow around obstacles (eg wall ties) in the cavity. In my own case whereever there was a breach in the inner leaf of the cavity eg for piping, or at the top of the cavity in the attic, the beads came flowing through showing, I hope that they were filling the space.
Finally, I discovered when the installer drilled the outer walls to pump in the beads that the polystyrene boards were in fact missing in places, especially at awkward corners or towards the top of walls where the builder clearly could not be bothered to cut pieces to fit. The beads filled these gaps.