The big issue here seems to be it's not possible to raise the CPU clock generator frequency above 200 MHz with this board due to BIOS limitations. So, why not build an external circuit or generator to replace the onboard signal to the CPU, instead of relying on the board's PLL hardware? This technique has worked well for me in the overclocking of numerous video game consoles and would likely be applicable to x86 hardware as well.

Generators exist to produce a squarewave of the required amplitude at any frequency, and so do lower-cost solutions like VCOs and fixed-frequency crystal oscillators. You'd just need to sever the clock trace to the CPU and attach your own signal. I don't imagine this would be a problem since the CPU reference frequency seems to be arbitrary and distinct from that of any other component on most overclocker-friendly boards (in the pursuit of separating FSB, PCI, AGP, PCI-Express, and other buses so a rising frequency on one won't interfere with proper operation of the rest.)