The rule regarding slings has probably never really been questioned or pushed into change because of the reasons I gave earlier. Most top shooters don't think it gives a great advantage because of the different angles etc.
I really wouldn't spend a lot of energy on this. I really don't think it's going to make a great deal of difference to you having the clip on option onto a band on your bicep. If you want to try one then do as Simon Ayers does and have a loop that you can put at different positions on your arm.