For my wife's 30th birthday party, we had a Doctor Who inspired larp. She got to play our Doctor stand-in. You can see the full suite of materials made for the game over here, if you're interested.
We only have two smartphones in our group (a third phone and its owner unexpectedly showed up at the last minute, but character sheets were already being passed out, so he didn't get to use his in the game). This meant that the larp had to have a limited number of people with access to the special senses. My wife's PC got one phone so that it could act like her sonic screwdriver. I debated for a while whether the other should go to our other timelord standin character or to the ancient evil alien PC, finally siding with the latter.
We used QR codes in a couple of ways. Every meaningful prop in the game had an attached to it that could be scanned. This scanning tended to give additional information about one of the plots or mysteries. For example, scanning the Quantum Flux Doorway revealed that it was a dimensional portal generator, not a device for teleporting between two terrestrial locations like the company was saying. Scanning the scientific test's data revealed that two immaterial beings passed through the portal they created. Some of this information could be gathered by other PCs in other ways: A supergenius alien could quickly skim the financial records to determine that the company was hugely in debt to a mysterious investor. But a normal human spending a few minutes and consulting a GM could get the same information.
On a similar note, one PC had a sixth sense for unusual electromagnetic phenomena. This manifested by each name badge and item card having a code letter on it like so: [B]. If the letter was a vowel, some weird energy patterns surrounded that entity. If a consonant, it meant that the subject was totally mundane. This is sort of like a low-fidelity, low-tech equivalent of the QR codes I stole from some MIT Assassin's Guild larpers. It also gave a second bandwidth worth of information: that information couldn't be encoded as more QR codes without it being accessible to my wife's PC. But simply having the letter visible to all but the meaning only apparent to one PC meant that that character got special information useful to he alone.
QR codes were also used on every PC's character sheet, for a tricorder-like medical scan. If you scanned the code on the PC, it told you if they were human, alien or a human with unusual brain patterns (insanity or possession being the culprits there). When making the PCs, I realized that I had to change the wording on the text a little bit each time, so that the QR code looked different to the naked eye. If I just copied and pasted the same text for each of the three possibilities, then you could glance at a sheet and see that two of them matched and one was different. So each scan had to be personalized to the PC just enough that the QR code looked different each time.
The medical scan could have been made more important or relevant, or expanded in different ways. For this game, there were only three important states, but you could build an entire larp around a doctor diagnosing patients using QR codes.
The last way QR codes were used was for locked doors. Three offices int he building were initially locked. Each locked door had a sign indicating which key was needed to enter. Each sign also had a QR code on it. If you scanned the QR code, then the text said the door was open and you could enter. Thus, our Time Lord's sonic screwdriver actually unlocked doors, just as it should.
One PC, Mr. Smithee had the key to his office, but the other two offices were locked. Dr. Kerensky had accidentally lost her keys, while The Clockmaker had stolen Mr. Manciple's keys.
The locked rooms had a few effects on the game: it helped pacing by cutting off certain areas of the game initially and placing an obstacle in Manciple's way. It spurred on interactions between PCs: Manciple and Kerensky asked around if anyone had seen their keys, Spelvin learned some of Manciple's secrets, and my wife's PC offered to give access to the locked rooms to certain PCs in exchange for their cooperation. Finally, it helped reinforce the idea of barcode scanner as sonic screwdriver, which reinforced the feel of my wife being The Doctor. Which was the whole point of the game.
Ironically, Mr. Manciple had a barcode scanner, which I later realized invalidated his being locked out of his office. But I don't know that he ever learned that the scanner could unlock doors. If I ran the game again, I might give the scanner to someone other than Manciple, Or eliminate the missing key matter anyway.
In practice, things worked very well. Much as I hoped they would. Occasionally, it would take some wiggling and effort to get a QR code to scan properly. But that worked well for a larp setting: it meant that the player had to scan surreptitiously or explain what they were doing. And it meant that some scans were quick and easy and others were difficult to get a bead on, which seems sort of fitting for The Doctor scanning something.
Sometimes the players were in too much of a hurry to read carefully the text a QR code gave them. My wife scanned and read the same Test Data code several times, but never noticed the part where it clearly states "Two immaterial aliens beings came through the portal to our world". But that's more her problem than it is a flaw in the system.
Making even moderate text started to get unwieldy. More than a sentence or two and you start to get pretty big QR codes. Bigger codes are harder to scan. And the scanner software that came on my Android phone has a weird scrolling function if the text is too long. But this was a surmountable problem. For one thing, it meant that I had to be more concise than It normally would be.
So the QR codes did exactly as I hoped that they would. They added a nice component to the larp, and provided secret information to some PCs while still letting the other players acting as normal. Best of all, it was easy to implement an required no GM intervention in game. I just had to use an existing website and a program that came free with my phone, and it worked like a charm.