I've repeatedly wished for a mechanism integrated into a phone that allows displaying one or more documents (e.g. QR codes, tickets, some other form of document to be displayed) while having the phone otherwise locked (in lockdown mode, so a password is required to unlock it). That would make it much safer to display a QR code on your phone without the net effect of having your phone unlocked when going through security.

Until then, I'll continue to print such things out on paper.