Saturated salt solutions produce defined humidities over them at defined temperatures. You could circulate the air in the autoclave over such a solution, chosen to give the humidity you want.

Various salt solutions is how you calibrate relative humidity sensors. That process can be quite accurate, but not for what you're doing. First, "relative" humidity is meaningless at temperatures above the saturation point. Moist air properties (e.g., ASHRAE) presume near 1 atm pressure and T

If the materials in the autoclave do not adsorb or desorb water, one way is to start with adding in the autoclave the exact amount of water that will result in the required relative humidity at the final temperature and pressure. Saturated vapour pressure vs temperature can be found on https://en.wikipedia.org/wiki/Vapour_pressure_of_water .

Saturated salt solutions are also an easy way to stabililze the relative humidity (see https://www.engineeringtoolbox.com/salt-humidity-d_1887.html or https://nvlpubs.nist.gov/nistpubs/jres/81A/jresv81An1p89_A1b.pdf ). Unfortunhately, I did not found values for high temperatures.

Thank you all