You might be better off seeing an independant girl who doesn't have the rest of her shift to fufill afterwards. In my past experience working at a massage parlour, getting my hair wet was an issue because it takes too much time to dry and style it afterwards, which we just don't have. I think most spas allow 5-15 minutes inbetween clients which is never enough time to re-do our hair. Just some insight Another suggestion might be to call and ask over the phone which girl might be comfortable sharing a shower with you as her last client at the end of her shift, when re-styling her hair afterwards wouldn't be as much of a time-pressure issue.
Good luck!