Recent FreeBSD 8.0-CURRENT snapshot ISO images are with BTX 1.02, which allows CF-R4 booting from USB CD-ROM drives. So I tested the following procedure for the 6.3 and 7.0 releases and the both releases were successfully installed:
- boot from a CD-R(W) with 8.0-CURRENT-200805-i386-bootonly.iso (or later)
- exit the boot prompt screen to the loader prompt (by pressing 6) before the 8.0 kernel boots itself
- enter unload to clear the loaded (8.0) kernel
- replace the CD-R(W) in the drive to the target installation image (e.g., Disc 1 of 6.3/7.0-RELEASE)
- enter boot-conf to load the target kernel and install
I think this is the compatibility problem of BTX. I also think this is so far the easiest way to boot from the USB CD-ROM drive for CF-R4, because generating ISO images for previous releases with the compatible BTX loader will be a big hassle. You may give it a try if you want to boot from a USB flash memory.