At that temperature I would be more concerned about the viscosity of the grease in the clutch disc splines, rather than the brake "hydraulic" fluid.
Maintenance of this fluid is cheap and easy, however, and makes a great starting point to either confirm or eliminate that part of the clutch system as a culprit.
Re-greasing input shaft splines is quite a bit more involved.
You also might try and vigorously work the clutch pedal with the engine off. Does this change the feel after a few minutes of pumping? Also, how do the gear changes feel at that temperature? Does it seem like somebody put syrup or peanut butter in the transmission instead of the normal "summer" feel?
My '76 FJ40 clutched and shifted like it was under water when the temperature was very low, but that was a long time ago at -5 to -15 Fahrenheit, with primitive single viscosity lubricant. -10 C doesn't strike me as particularly cold, but the feeling is subjective - and the "clutch effort" perception you are sensitive to might not be a problem; just a keen observation.
You could gently heat the clutch master cylinder (no flames, something like a hair dryer) and see if that makes an immediate improvement. Or better yet, just replace the fluid completely (bleed if possible) and see if that makes an improvement. The latter approach accomplishes maintenance you need to do anyway - especially if the fluid is original at 163K.