How to get readonly access to mysql files via g+w permission 750 or 640?
Well, I found online that the environment variable UMASK and UMASK_DIR must be set, regardless of what the command umask will output.

One must add export UMASK=0640 and export UMASK=0750 to the top of their startup script, immediately before calling mysqld_safe.

The leading zero is important to switch to the octal numbering system.

The first thing I tried was 0027, but that produced files with permissions u=rw,g=w,o=rwx so that is what prompted me to invert the permissions in the environment variable to 0750 (and 0640) to produce the desired results.

I hope my self-answer will help a future visitor. :-)