friend a dependent typename of a template class

I have 3 classes, Device, Register, and WriteOnlyPolicy, defined as such:

Device

class Device
{
public:

    template<class Register>
    inline void write(typename Register::value_type value)
    {
        Register::writeRegister(value, this);
    }

protected:

    // limit device-specific API calls to this function
    virtual void writeDeviceRegister(uint64_t address, uint64_t value) = 0;

};

Register

template<uint64_t ADDRESS, class ValueType, template<class> class AccessPolicy>
struct Register : public AccessPolicy<ValueType>
{
    using value_type = ValueType;
    using access_policy = AccessPolicy<ValueType>;

    static void writeRegister(value_type value, Device* device)
    {
        access_policy::write(ADDRESS, device, value);
    }
}

WriteOnlyPolicy

template<typename value_type>
class WriteOnlyPolicy
{
protected:

    static void write(uint64_t ADDRESS, Device* device, value_type value)
    {
        // convert from a value_type to a uint64_t...
        // Shift/mask as needed...
        device->writeDeviceRegister(ADDRESS, value);
    }
};

Register and WriteOnlyPolicy never actually get instantiated in this scheme, but I want the policy to call the protected writeDeviceRegister function of the Device class. I don't want that function to be public as I only want to expose the write function to users of the class, and not the device-specific implementation.

In order to allow that I want Device to friend Register::access_policy, but I can't figure out the syntax to friend a dependent typename of a template class. Is this possible?