It is permissible for a fasting person to use a miswak while fasting, irrespective of whether the miswak is dry or fresh. Even if the bitter taste of the miswak or its strong flavor is felt in the mouth, it does not affect the fast, nor does it make the fast makrooh.
Using toothpaste while fasting is makrooh due to the possibility of its particles reaching the throat. If toothpaste is ingested (goes down the throat), the fast becomes Fasid (deemed invalid).