Can you measure absolute specular reflection without a calibrated reference mirror?

Yes, and this type of specular accessory also can make variable angle of incidence measurements.

Specular reflection is unique to smooth “mirror-like” surfaces. While it is possible to make direct specular reflection measurements with an integrating sphere using a calibrated mirror for the background correction, it is better to use a dedicated specular reflection accessory designed specifically for that mode of reflection. These accessories can be either fixed or variable angle of incidence as well as relative or absolute in design. A relative accessory must use a calibrated mirror for background correction and is usually fixed angle of incidence measurement. An absolute specular accessory yields absolute (calibrated) percent reflectance values due to its optical design and is usually variable angle of incidence. Absolute reflectance accessories are more expensive than traditional relative reflectance accessories.