Yes, the legal issues are always a concern even when it is just for fun. Creating the functionality to link a profile to a unique QR code would make nice functionality to build in while developing the website. i tried to develop my own years ago but got a little over whelmed as i was the only coder and the language i new suddenly became unavailable for websites than hosted adult content. Below are some images i created for a sortof slave identification system that was never completed. The idea was that every sub or Dom would be able to print out a tag. The image on the front would be quickly identifiable as Dom or sub and general interest. The back would have some sort of code for their limits and owner and a bar code linked to their profile.