I think this is a great feature for mobile users. On many occasions I read an article on Internet at work and want to save the link to my mobile phone. Right now, I use Google Keep to save various notes or links, which are then available on all my devices. It might be beneficial for users if links posted into posts only can have a QR "hover feature" that allows you to quickly open it into your mobile. Even if this could be a feature specific for a certain niche of clients, I think is innovative and it will be available first on XenForo. Not sure if other forum software offers this already.
The ideal implementation is through a BBcode option, similar to [CODE=rich] functionality, i.e. URL=QR,domain.com.