Has anyone successfully got a SPFX Ext (app cust) running in a non-Dev tenant yet?

Cross posting from https://github.com/SharePoint/sp-dev-docs/issues/847

Has anyone been able to successfully load an SPFX Extension ApplicationCustomizer in a regular tenant (non Dev)?

Everything seems to build, and install fine, CustomAction is added, (i'll get 404 errors if I dont load the dist files, so I know it is registered), but Extension just doesnt run.

Per https://dev.office.com/blogs/sharepoint-framework-extensions-rc0-now-available