Nevermind - I just see m68k defines ARCH_DMA_MINALIGN to be four bytes. Should be safe for all that matters, then.
An example of a user space application that passes an SG I/O data buffer to the kernel that is aligned to a four byte boundary but not to an eight byte boundary if the -s (scattered) command line option is used: https://github.com/osandov/blktests/blob/master/src/discontiguous-io.cpp
Thanks - four byte alignment actually wouldn't be an issue for me. It's two byte or smaller that would trip up the SCSI DMA.
While I'm sure such an even more pathological test case could be written, I was rather worried about st.c and sr.c input ...