Yiiiikes. I did find an AMI BIOS file from a 915P board that has code for Cedar mill, but I don't think that would be of any help.
EDIT: I found this Fujitsu-Siemens BIOS which is for a motherboard on the 915 series of chipsets! It is Phoenix based and has Cedar Mill support. It MAY need the F65 CPUID microcode, though. It appears to have references to F62 and F64 microcode. I've included a ZIP file known as D2420 and it has a compressed OCF, decompressed ROM (which can only be used to extract via phoedeco), and the extracted BIOS in a D2420 folder. Also attached is my phoedeco output to help pinpoint the files.
EDIT: I found this Fujitsu-Siemens BIOS which is for a motherboard on the 915 series of chipsets! It is Phoenix based and has Cedar Mill support. It MAY need the F65 CPUID microcode, though. It appears to have references to F62 and F64 microcode. I've included a ZIP file known as D2420 and it has a compressed OCF, decompressed ROM (which can only be used to extract via phoedeco), and the extracted BIOS in a D2420 folder. Also attached is my phoedeco output to help pinpoint the files.