Time flies.
I am finally getting stuck into this project again and hope to make some major progress in the coming months, having a whole bunch of chassis underway. Just soldered the remaining parts to the regulator board for the +/-100V Reference Supply chassis, which has been sitting dormant on the shelf for over 3 years now. Seems to work as intended. The reference for the regulator is a good old LM399 and this thing wasn't designed to set any benchmarks for ultra low noise, but to just have a decent degree of stability and negligible mains ripple/hum. Drift under load over several hours of initial testing, sitting on my bench, has been well under 1mV.
Now that I have a new toy (Keysight 34460A) I can properly calibrate stuff and characterize all of my non-linear modules to a decent degree of accuracy.