I'm not familiar with that specific board, but in general lowering the FSB will result in an under-clock. You could run into problems if it doesn't enforce a divider though. If you're trying to slow the thing down for compatibility, you may also try disabling the cache (if that option is available). Finally, depending on the board, it may let you select a lower multiplier than whatever is stock for the chip.

66MHz is the slowest bus speed of the BM6 and your CPU got a fixed multiplier so you're stuck with disabeling the cache.

obobskivich, Kamerat,

Thanks for the replies and confirming what I suspected; I'm SOL with this CPU.

Fortunately, both Mendocino and Coppermine Celerons are super cheap on eBay nowadays. I can pretty much select any one I want for ~$5.

The lowest you could get with a Coppermine Celeron would be the 533A, or the 800 with an FSB of 66MHz also resulting in 533MHz.
If you don't want to use a Mendocino you could get a PIII Coppermine with FSB100 or FSB133 and underclock it, the lowest would be the 533EB with FSB66 giving 266MHz. I didn't check how cheap those are on US ebay.