NCLs official statement of:
"However, in certain ports of call which will be specified onboard, guests who have pre-purchased any of NCL’s beverage packages may be served"
To me implies there is no forewarning and guests will not be aware which ports you can use your beverage pack in until onboard.
Again this is my interpretation of the statement and as seen as they will not comment futher to clarify the language used, who actually knows.