Speaking as a spa owner here, understanding the ladies perspective. This is not concerned a standard booking and whilst you may not consider it an extra it would be gentleman like to offer your therapists a gratitude for such a service.

Maybe if this was discussed and she understood that a couple of hundred would be on the table she would be more open the the idea of someone caressing her naked body for their pleasure.

I was celebrating my birthday and I had to work on that day because it was the end of Easter holidays. Although my day was quiet, I received a tip as a gift from a lovely gentleman :) that gift was so heart warming, the kindest gift i could receive under those circumstances. You never know when you're making someone's day, or week ;)