Since Hanuman is avatar of shiva, can I offer Bel leaves as offering in Hanuman temples?
No,you cannot offer Bel leaves to Lord Hanuman. Of course he was the 11th avatar of Shiva,but he was only a part of Lord Shiva and he was not Shiva himself. So,you can offer Bel leaves to Lord Shiva,not Hanuman but you can offer Rui leaves to Lord Hanuman. You have to read the book of the rules of Lord Hanuman. If you offer Rui leaves to Hanuman,he will be very happy on you but if you offer Bel leaves to Hanuman,Lord Shiva and Hanuman both will be very angry on you. So,you have to follow the rules of Lord Hanuman.
There is no hard and fast rule that since one god is the avatar or amsha of another one, all that is valid for the parent or source god is valid for the other also. But Yes, You can use Bilva (Bel) leaves to worship Hanuman. It is mentioned in Parasara Samhita Hanuman Charitam Chapter 49 (एकोन पञ्चाशत्पटलः) Sloka 7 that Bilva leaves are used in Puja of Hanuman Ji. The sloka is as follows:
तिलाक्षतान् श्वेतवर्णान् शालीया नव्रणानपि । बिल्वामलक दूर्वाश्च अरविन्दवनं तथा ॥
बिल्वामलक means Bilva and Amalaka (Amla) Leaves can be used.