Natively, no the support isn't there, but the hardware may have been set up to allow SDIO, that has turned out to be the case with the c860, there are no drivers, but the hardware is functional. There is a SD wifi solution floating around here that has had a sdio driver coded for it.
Unfortunately it may be hit or miss, as Sharp didn't even connect the sdio line in the 5x00 series, not sure about earlier clamshells tho.