There are pumps in service that are intelligent enough to accept QR codes (like Dresser Wayne & Gilbarco)... Mobile uses this type of technology for their speed pass system... several offer loyalty programs that are fairly sophisticated...there are also pumps that can be retrofitted with a side box for passing payment codes to the POS system.
QR displays and NFC would be great ways to initiate a bitcoin transaction at the dispenser. Too bad newer models with those capabilities are very rare to find in the wild. In US markets anyway. Perhaps with all the changes we'll no doubt see with the conversion to EMV will be a catalyst for dispenser upgrades across the nation.