It appears that “bootaux comple” is significant.
My underlying issue seems to be that the elf image was not being correctly loaded. For some reason which is unclear, the bin was also originally not being loaded correctly.
However (and this name change may be coincidence) loading the bin as “m4image.bin” has made it work - I get messages on the M4 serial console, RPMsg is working, and U-Boot reports “bootaux complete”