Thanks for the suggestion!
While http://localhost:80/Host/HTMLEditorManager?portalid=0 didn't get me there, it did give me a clue where to investigate.
As noted, clicking on the "Open HTML Manager Editor" button got me the Page Not Found error within the Persona Bar admin (slideout).
The suggested URL got me a Page Not Found from IIS (completely outside of DNN).
Playing around some, I found the problem might not be with the undefined portalid, but with the "Host" in the URL.
I substituted "DNN9" (the alias on my machine) for "Host" in the URL (leaving portalid=undefined) and successfully navigated to the HTML Editor Manager page, in a full page, not within the Persona Bar.
I have this same issue with both the upgraded site mentioned in my initial post and in a fresh DNN 9 install.
I don't know if this provides any clues as to what is going on...evidently something peculiar to my machine.
Oh well, your help got me to a workable workaround.