Just because there is (or will be) a dedicated site for a specific topic it doesn't mean that any related question is off-topic for another site.
I'll give an example from the Stack Overflow side of the network:
Questions about wordpress or drupal are accepted on Stack Overflow even though both have dedicated sites:
When the lines are a little bit blurry but one topic is encapsulated in another it's totally fine to ask in either site as long as you're not cross posting and duplicating content.
So in short it is acceptable to ask your coffee question on the Cooking.SE and even once the coffee site launches, it would still be ok to post your question either there or on Cooking.SE.
Regarding the OP's comments related to closing as duplicates once the new site opens:
It is possible that once the coffee site is up and running that the Cooking.SE community will decide to purge all coffee related content and migrate it although as I said before, just because there is a "better" place to post such a question, it doesn't mean that it is automatically "off-topic" for it's current location. Closing a question as a duplicate on a different site doesn't really happen - what can happen is that the post is migrated and then closed but, again, it all depends on whether the community decides to take these actions.
Ethically, as long as your intention is to add to the collective knowledge of the community - you can't go wrong.