Just found what I believe to be your message on the lists, have you considered building a kernel with debug messages enabled?
It may give some additional information to the developers, and if you can boot using a USB device there is no need to through the process of building a RAMDISK kernel.
option WDCDEBUG
Alternative, you can
disable pciide in the UKC and the kernel will attempt to use the legacy ISA
wdc(4) driver, it will be somewhat slower.
Good luck, just curious.. but does the BIOS/chipset not allow AHCI mode? that would avoid the pciide(4) driver usually.