I had pretty much thrown in the towel on the prospect of ever running Pharo on my Pi 400 and ironically settling for Squeak led me to the solution. The official Squeak 5.3 bundle for 32-bit ARM segfaults, which led me to the (as of this writing) latest virtual machine release page. I was able to get Squeak to run from separately downloaded image, change and source files with the "stack" VM release. (The "cog" release also segfaulted.)
Looking through these releases I saw there was also a VM for Pharo, same architecture. (By the way, that release is NOT in any of the release files for Pharo's official pharo-vm repository, so looking there would have gotten me nowhere!) I was intrigued and attempted to do with that Pharo VM what I did with Squeak. More segfaulting.
After some Google searching, I read that instability in the behavior of the VM
over time caused the same issue so I tried an earlier release. It almost
worked but for a complaint about the VM being too old and an error with
LGitLibrary not being able to find a shared library. Further research into
that problem revealed that libgit2.so.0 (or alternatively just libgit2.so)
was missing. What ended up solving the issue was going into
lib/pharo/5.0-201901172323 and linking the system library like so: ln -s /usr/lib/arm-linux-gnueabihf/libgit2.so.0.27.7 libgit2.so.0.
The other issue about complaining of an old VM was solved by using a somewhat newer VM. I tried one a few releases back and it solved both problems.
And, actually, after having done a bit of research, getting the newest VM from here (very confusing!) and combining it with the latest image and ancillary files worked even better! (Symbolic link still needed as before.)