We add the option to specify lead and trail times on bookings in bug 34440. It would be a nice addition to add the option for some users to opt to override the rules at booking time in some cases. Example; You add a trial time rule of 3 days which should account for tardy patrons in your organisation returning the items at the end of their booking period. However, you're making a booking for a patron who you trust to return the book on time so you want to, in this case, allow an override to reduce the trail time.