Lacking occ the next best option would probably be to directly add it to the database:
Something like:
INSERT INTO oc_appconfig VALUES ('core', 'defaultpage', '/apps/files/extstoragemounts', 4, 0)
Assuming the default dbtableprefix is in use (oc).
This could be done using any db client (e.g. phpmyadmin, mariadb, mysql, etc.)