I've managed to find a workaround to get these disks to be detected and mount. The initrd needs to be remade using the options --with=scsi_mod --with=sd_mod --with=sata_sil. Adding itthese modules to /etc/modules.conf isn't enoughdoesn't make a difference.
mkinitrd --with=scsi_mod --with=sd_mod --with=sata_sil /boot/initrd-$(uname -r).img $(uname -r)
There is still one more step though. modprobe sata_sil still needs to be called after the system has booted, either manually or through rc.local. The output is also different from before, with additional output showing that the drive has been attached.