Actually, Thorn was running a C566 on a BM6 if I remember correctly, which is definitely a Celeron PPGA only designed board. If you check ABit's website, the latest BIOS update supports 566 and 600 Celerons.
I don't know what this proves, but there are definitely PPGA only mobos and slotkets that support FCPGA Celerons. Whether they had the foresight to engineer in some FCPGA compatibility or if the C2s up to 600 have some backward compatibility I can't say.
What we discovered though was that the C2s were more overclockable on PPGA only slotkets than they were on FCPGA slotkets, at the time. Eug just had a nice thread on more recent slotket developments about a month ago.