Okay, so to clarify you just want a buck regulator which regulates to 300VDC, but when the input drops below that it's acceptable for the output to droop too and lose regulation.
You won't have to build your own pwm controller, but you are going to have to make your own gate drive circuitry, since I'm not aware of any controller which can switch 450V. Unless it's an external PFET. But you'd probably be much better off with an N channel IGBT, so you'll need a high side gate driver, and a general purpose PWM controller. If you use a bootstrapping gate driver, you'll need to limit the duty cycle to <100%, so you'll get a bit of voltage dropout no matter what, but it doesn't sound like that's a big issue. The only potential issue I see is if you need current limiting, which is tricky to implement with high voltage buck regulators.