For the value list, you need to setup a relationship that only shows empty beds through the relationship. Then you setup your value list to only show related values via that filtered relationship, and your dropdown will only show available beds.
A diagram would be possible too, but more work. There's some solutions out there that do just that, here's some examples to inspire you:
even this: Sudoku for FileMaker - SeedCode
Check out SeedCode "airline seating example". That has the conceptual features you desire.