I have finally found out that in the GPO update the timpani can be muted (thanks DarwinKopp!). But: It seems that I can´t mute them to a musical length, instead I only have the choice between sustained and the one existing "damped" sample. What if I want them to sound exactly one quarter note?
Hmmm, that's a technique of the movement of the hand across the head of the timp. I can remember just practicing dampening techniques for different note duration's but I think you could achieve it by using the combination of the two. Am I mistaken or isn't the dampening for timpani done with the pedal control? If so, you could hold out the quarter note to full duration then place the dampening at the end. I wish I was at my keyboard to find out if this is so or not. Hmmm.