So, you'll have to please excuse my techno-peasant-ness here, but a few months ago I had a magazine with QR codes and it did not require a smartphone. I used my dumbphone to take a picture which I then sent as a picture/text message to a specific telephone number then it sent me some text msg spam back.
Now, I'm not exactly sure what you do for the telephone number and the automation of sending back a text msg, but I hear that the google voice allows you to do some cool stuff and I believe you can get a free phone number from them with voice mail and all sorts of goodies. Maybe you wouldn't even need to send a text message to the user. You could have instructions recorded on the outgoing voicemail message. This would include a lot more user who don't have smartphones, but do have phones that they can take pictures and send text messages with.
And one last thought, If the size (and I guess intricacy and required picture clarity) of the code is dependent upon the number of characters that it represents, why not encode a relatively simple/small number that represents a larger message in a data base. For instance, instead of trying to encode coordinates, you could encode the number "12" which when texted to a specific phone number hits a data base, and "12" points to a file that hold the actual co-ordinates which get texted back to you.
I'm not a techie so I hope that wasn't just a bunch of blabber.
