Some Leaf drivers wave, but I don't think it's standardized. I've seen, and given, what we might call a Porsche-style wave (a hand partially raised from the steering wheel) and a flashers' wave (blinks of the high beams by pulling on the turn-signal stalk twice).
If we're going to standardize, I suggest the two flashes of the headlights, since they're electrical.
I've also waved at a few Teslas, which I see more often than Leafs, but they don't wave back.
2017-12-16 New Prius Prime
2018-02-21 Used pearl white 2015 LEAF SV Mfd 2014-07 In-service 2015-02-27
2018-02-21 26,938 mi AHr=60.22 SOH=96.98% Hx=90.73% 1 QC 988 L1/L2
2018-07-01 31,601 mi AHr=58.05 SOH=93.48% Hx=87.03% 11 QC 1223 L1/L2