OK, after some more digging, I found a way to apply a _custom.less by dropping it in a neutral directory (lib/web/css/source) and then making a scary looking import in each place preceded by ../../../../etc
just shakes head
c'mon magento. You can't edit the custom css without a third party plugin or a full static content deploy in the backend. You can't see the css in the content editor. I sure hope Adobe fixes some of this.