In my atypical Block List, I wish to add a Block that would be pre-populate with default content. That content would single property using another Block List property editor. I haven't a clue how I might go about achieving that. Anyone ever managed anything similar before?

I've created block list items programmatically - there some docs on this here -> https://docs.umbraco.com/umbraco-cms/fundamentals/backoffice/property-editors/built-in-umbraco-property-editors/block-editor/block-list-editor#creating-blocklist-programmatically There are also Editor Model notifications which allow you to manipulate a model before it's sent to the editor - https://docs.umbraco.com/umbraco-cms/reference/notifications/editormodel-notifications It will be complex, but some combination of those might get you there?

Sorry not to have responded sooner. Thanks for this. EditorModel Notifications is the answer.