I never figured out the desired default log location, but this worked for me to change the log location to /var/log/cas (a directory I made which is owned by tomcat):
1) Build the default overlay war (I've built it in /opt/cas/cas-configserver.orgname)
2) Copy "/opt/cas/cas-configserver.orgname/build/overlays/bootWar/cas/WEB-INF/classes/log4j2.xml" to "/opt/cas/cas-configserver.orgname/src/main/resources/log4j2.xml"
3) Edit the new file, changing the log location on lines "12 and 13":
3a) I changed fileName="casconfigserver.log" to fileName="/var/log/cas/casconfigserver.log" on line 12
3b) I changed filePattern="cas-%d{yyyy-MM-dd-HH}-%i.log" to filePattern="/var/log/cas/cas-%d{yyyy-MM-dd-HH}-%i.log" on line 13
4) Rebuild the overlay war and run that.