Can I still claim office rent as an employment expense even when my employer's T2200 has indicated that my contract does not require me to rent an office?

No, you are only allowed to claim employment expenses as outlined in your T2200 

Thank you for your response. My employer does require a home office and has indicated this on my T2200. However, our house is not suitable for a home office so I've had to rent another office space. This is where I am stuck with allocating this expense.