When activating Howlpack Piper to put a Werewolf from hand into play, after the ability resolves, the ability is still shown to be on the stack.
Priority is given as if the stack was empty.
Any game action causes the ability to disappear from the stack and the Piper to untap.
When simply pressing 'OK', however, the game prompts me to put another creature into play.
I have found that this only occurs with Werewolves, not Wolves.
Using this, it is possible to put any number of Werewolves from hand into play.
Testing was done in a 1-player game, so I do not know how an opponent can interact with this.
Screenshots attached (I forgot to take a screenshot after putting the first Werewolf into play).