Turns out the BOINC you install via the 64-bit container has a different data directory, that you can only find when you close the 64-bit container.
You have to set your parameters to
/var/lib/machines/debian-buster-64/var/lib/boinc-client/
instead of
/var/lib/boinc-client/
This doesn't save the day though, I run into permission troubles. Heck, I even get permission troubles in the 64-bit container while sudo-ing!
Guess the only valid option to run the Ralph WUs for 64-bit Linux/ARM is to have a fully 64-bit OS running (64-bit Balena/ARM? ArchLinux/ARM64?)