Unfortunately if Wix Bookings doesn’t have this as a feature / function / API ---- then it cannot be done. We would not be able to use Wix Code either because it is not part of their API: https://www.wix.com/code/reference/wix-bookings.html
This means, either submit a feature request or use something else that gives you this ability if it extremely important for the client to have this option. I do not know of any 3rd party app that has this functionality, so I cannot make any recommendations.
You can mostly build this entire thing from scratch using Wix Code. It is advanced coding and advanced logic, but #totallycodable.