Full disclosure -- also not an EE, that was only my minor. Also not a HW designer much less a power supply guru.
I'm sure my post came off more defensive than I intended, but then again, a rebuttal of almost any sort easily tends in that direction. I think your questions and goals make sense. It will be interesting to see what other's would have done. If there is information that would make the alternate design exercise easier or more factual, I'll do what I can to get the info to you. In that spirit, here are the
links to the specs, user manual, etc.
Here is a page from team 358 that did some pretty extensive independent testing of the entire system and details what happens as the voltage does the limbo. They took the roboRIO to under 4V before it rebooted and the radio under 3.5. Not your 3.3V goal, but in the neighborhood. And as documented and designed, it is staged, which introduces the brownout condition. Some things start to disappear at about 6.8. During alpha I believe it was often rounded to 7V.
Beta team 358 Testing of Power Management
Here is a direct link to the documentation of the power management stages.
roboRIO User Manual and the details are on page 6 and 7.
Just to round out the power discussion, the other requirement, the one that was given a higher priority, was to allow for any pin to short to any other pin with no damage. We regularly rake a screwdriver across exposed pins to demo this. The power tab of the DS will show you which rail is shorted. So detection and protection against over voltage and shorts was deemed critical. The system will also tolerate being plugged into the older 24V PD without damage. It won't operate, but isn't damaged and indicates the issue via LEDs. By the way, having USB cables with exposed ground shield on a robot with high current 12V exposed connectors is also challenging.
Oh, and 6V is used to power servos.
Greg McKaskle