There is an ongoing discussion about an enhancement related to your request (latest is at PP-507: Add support for filtering nodes as per the job request). Depending on your high-level use case (the goal you are trying to achieve), it might provide a better solution (in a future version of PBS Pro). To that end, it would be really useful if you could take a few minutes and describe your use case in more detail.
Ideally, describing what you are trying to achieve at a higher level (above the implementation level). For example, what kind of application is being run? Are you expecting it to execute faster when run on fewer nodes – is that the main goal? Or, are you trying to bin-pack the machine, using fewer nodes, with the goal of better overall utilization? When you say “if not available then …”, do you mean “if not available when the job is submitted”, or “if not available when the PBS Pro scheduler tries to start it”, or “if not available ever” or ?. Basically more background and high-level goals would help in making and refining design decisions for the future enhancement to PBS Pro.