I appreciate all the replies, thanks.
And some really cool tools here, so I hope these come in handy for me and others in the future.
However, no, still not what I'm looking for. It was much simpler, even without a UI, I think - probably a modifier.
Take an object, select some faces and click a button to have them replaced by an object. No idea how 'smart' or adaptive the process was, but it seemed straight forward and good for simple application.
I'll keep hunting. Thanks again,