Tested at Sunsong Ranch.

oUF.Tags.Events["RealTest"] = "UNIT_NAME_UPDATE"

oUF.Tags.Methods["RealTest"] = function(unit, real)
    print(unit, real)
end

-- Returns
"vehicle nil" -- enter vehicle
"player nil" -- exit vehicle

You need to set fontString.overrideUnit to true for it to pass real unit.

Oh, right. I've used that before and completely forgot about it. Thanks.