It is quite normal that a customer would like to have Maintenance done on the instruments on specific week days / hours. And that those will be different then the normal business hours for getting instruments repaired.
So it would be great if we can have field to maintain this information on the instrument (perhaps header) and that these days are being considered by the Propose schedule and Recommend Resources. Also the validation rule for preferred days / hours should consider them based on the days and hours but also on the WO type (Maintenance, Repair etc)
What is the underlying problem do you intend to solve with this idea?
Less re-scheduling, efficiency, avoiding customer dissatisfaction
How is the problem being addressed today, if at all?