You can get wind and rain resistant (Montane FeatherLite H2O and TNF Hydrogen for about 3/4 oz) however if you want waterproof I think that the TNF Triumph smock at around 5.5oz is the lightest.
Don't know if this is light enough but have a look at the Montane Atomic jacket, 8 oz for the M size.
Mine is an older (discontinued) smock version. It can take hours of heavy rain. They pack very small.
BTW Ethan, you should try the gear forum for this one.