I'm not sure if you're looking for something clever or simply something that will fill in the blank, but if the latter, I would suggest hydrated (or, as my kids used to say, hyderated.) From the Oxford Dictionaries:

hydrate

 

VERB

 

[WITH OBJECT]

 

1. Cause to absorb water.

‘a powerful moisturizer that hydrates the skin for up to twelve hours’

Now, while sipping some water would not technically hyrdrate your lips, but given that one's whistle is a little more abstract (or, I suppose, the space between your lips when whistling), I think it suits.

In your example, it would be:

An appetite is whetted until it is sharp; but a whistle is wetted until it is hydrated.

I'm not sure if you're looking for something clever or simply something that will fill in the blank, but if the latter, I would suggest hydrated (or, as my kids used to say, hyderated.) From the Oxford Dictionaries:

hydrate

VERB

[WITH OBJECT]

1. Cause to absorb water.

   ‘a powerful moisturizer that hydrates the skin for up to twelve hours’

Now, while sipping some water would not technically hyrdrate your lips, but given that one's whistle is a little more abstract (or, I suppose, the space between your lips when whistling), I think it suits.

In your example, it would be:

An appetite is whetted until it is sharp; but a whistle is wetted until it is hydrated.