Still, 50 of 'em ? I don't think they're available in such a volume from a single source.
Beside, if 1.1GHz CPUs could do the job, I don't see a problem with running a Sempie underclocked due to FSB limitations, the lack of BIOS support is much more of an issue IMHO.
Your best bet would be to try putting a Sempron on one of those mobos and see if it works.
Regardless, I don't think it is smart to purchase those elderly mobos in bulk, even if you manage to find enough CPUs and memory to pair them up with, I doubt that you'll be able to sell them.