Some of the locking happens in the PLL chip. Whether it's the BIOS that activates this, or it's burned in at the factory, or hardwired IDK. There's a user from China at BIOSMods.com named Genius239 that does some Dell BIOS modding. He modded an early Optiplex/ Xeon BIOS for me to avoid digital signing. Perhaps if you start a thread there, or PM him directly you can get one of those guys interested. I know on the PLL chip there is a locking pin that gets a locking signal (input) at startup, and then has an output for the USB bus after that to avoid spoofing the signal. Perhaps the BIOS chip has a similar scheme, or may be hardwired to activate a locking feature to do this. The datasheet for the chip itself may reveal this.
One approach could be to mod some memory modules for reduced latency using RWEverything instead of raising the bus speed. If you can get a low Voltage module to call for standard Voltage there should be some headroom there. On the one PLL I investigated there was one setting to lock the PLL, but another to select which FSB it was locked to. No one here has tackled Dell BIOS modding. It's there to be done. I wish I could be more help, but it's outside of my very limited experience. I do TS overclocking because it's usually very simple.
Did you change any settings when you flashed the BIOS, or just try to unlock the menus?