I seem to have a problem when trying to use a lzma pre-compressed payload in buildrom in combination with an etherboot (or filo) payload. Things work fine with a kernel or LAB payload.
I've attached the LinuxBIOS Config.lb I used, and the boot output for etherboot. I recall having similar problems with a filo payload.
If I disable CONFIG_COMPRESSED_PAYLOAD_LZMA and CONFIG_PRECOMPRESSED_PAYLOAD in Config.lb and disable the LZMA compression in buildrom's kconfig the Etherboot payload works just fine.
I just tried it with serengeti_cheetah, and it worked for me with just a fallback image. I've attached my Config and bootlog.
I noticed that you have CONFIG_COMPRESSED_PAYLOAD_LZMA and CONFIG_PRECOMPRESSED_PAYLOAD set in your Failover image, even though there is no official payload.
I couldn't see anything else, but I hope this helps.
Myles