From my comments, with credit to @Sean and TimWescott (Since they have not yet answered).
If you only want to knock the peak voltage down by ~100V, there is no need for a super high voltage regulation system. Typical modern linear regulators really only require a "reference" for ground since they often sink far less then 1mA. Look for low "Quiescent Current" regulators often designed for low power.
To create a virtual ground reference you could do it two ways:
- Use a resistor voltage divider with an RC filter to create a short term steady ground based on the input voltage. This is ideal if you have good long term voltage stability from something like a SMPS and you want to reduce ripple.
- As Tom noted, you can use a stack up of Zener diodes to create a known ground potential, but you create a risk of over voltage if your incoming supply has a large variation, also there will be some noise from the Zener diodes, but probably still cleaner then option 1.
NOTE: Values in schematic are only for reference, they need adjusting based on your voltages, currents and regulator of choice. For the regulator, check out the TL783 or LR8N8, both are good for 450V.
simulate this circuit – Schematic created using CircuitLab
simulate this circuit