The installer will expose some tunables to allow defining an alternative OS image. The current WIP looks like

image:
  offer: ocp-rhcos-worker
  publisher: redhat-rhel
  resourceID: ""
  sku: ocp-4-free
  version: 4.9.2021111600
  purchasePlanName: ocp-4-free

The newnew field is the 'purchasePlanName', but as Hive's current MachinePool API doesn't expose any of these fields, we'll have to extend the API to allow for defining sets of machines that are ultimately run with these non-default OS images.