I assume this is true even if that was the last possible action for the round. In other words, the active player is currently using their last non-free action and they have no free actions they could declare after the current non-free action.

OK. So it will be from now on when I'll judge, then.
Thanks for the clarification, Harpua

No problem.

This hole has now been closed. Venom Pump can no longer be done in the middle of an action as the effect now requires a free action to activate. (And actions cannot be done in the middle of an action unless the effect indicates otherwise.)

From the Player's Guide: