Actually, you don't show the airport personnel the barcode. There is a reader. You pull up on your phone the bar code and place it in front of the reader. The agent has a display which confirms that your bar code has been read, and displays your name and flight number/time. The agent compares that name to the government-issued ID you have handed over, to verify your identify.

Using an iPad to open the email/link, and resizing the barcode to fit in the window of the reader (which is roughly the size/shape of a current mobile phone), should certainly work.