Solved this by patching frappe-bench/apps/frappe/frappe/public/js/frappe/views/reports/report_view.js version 15.12.0, replacing function setup_defaults() by:
Customize a doctype, View Settings, Default View to Report. Adjust that doctype’s default report view (columns to display via Pick Columns menu) and Save As custom report named “< doctype > Default”, e.g. “Work Order Default”. Then this report will be shown by default for that doctype for all users.
And to prevent the default fields (name, docstatus) from sticking to the beginning of the report, in the same file, function set_fields(), move the line:
down to be the last line before the final one in that function:
this.set_default_fields();
Since these changes modify core Frappe code, it will be overwritten by the next update. So, need to find a way to restore them (git stash/pop?) and also copy to production.