Unfortunately the reverse breakdown of 1N4148 diodes is only a little bit above 100V. The solution (as seen above) is to simply run a few in series. I used a ceramic capacitor on the input, and an 0.47uF X2 rated filter capacitor on the output. Not really designed for high voltage DC, but it did the job! I was able to squeeze 426Vdc out with effectively no load (10M ohms).
Lowering the amplitude from the signal generator gave me the 300V max the 3478A will take:
Conclusion: It isn’t too hard to generate high voltages with readily available parts, and my 3478A works as expected. I didn’t even electrocute myself! Seriously though.. a little caution goes a long way.