The only ways I've seen it work are as follows:
1. Use of a program like softFSB or speedfan to change the FSB from within windows, you must know the exact clock chip model number or you could do some serious damage to your mobo.
2. Discovering that the mobo is in fact some overclockable board with the jumpers removed and soldering some wires on if there is the space (very rare)
3. Discovering that the mobo is just a rebadged board with a modified BIOS, which, with a little hacking could be reflashed to the original one. BEWARE: While HP and Compaq have done this in the past, sometimes the boards have been sufficiently different to make this technique kill your board.
I don't expressly reccommend techniques 2+3, but if you have the knowhow, and the balls, then you could try it, but understand the risks involved.
Number 1 is ok, so long as you get the chip number and its supported.