So indeed it is the DDR4 modules I’ve been using that’s the problem. A Micron 32GB 2Rx4 doesn’t work among several I’ve tried. A Samsung 8GB stick in a 1Rx8 config does finally result in the DDR coming up. It’s unfortunate that they don’t output anything prior to memory training (I assume they’re running from cache in an OCM config while training, it’s not rocket science to output something on the UART). Anyway, at this point I am able to load the LinuxBoot[0] and get the Fedora 38 image booting.
[0] This should be UEFI. LinuxBoot is a poor choice for those moving from existing ecosystems and will only make the process of building distros more complex than it needs to be.