Interesting challenge Howard,
We recently invented something similar as part of our Magic Reports solution. One of our clients wanted a such a "public to Enterprise" link to a portfolio specific version of our Hotsheet solution in order to then post that link under its corresponding Portfolio channel within MS Teams so the (non-Workfront) users within Teams could then simply "see" the report without having to log in to Workfront (not to mention set all of them up with requestor licenses...same deal as you).
To solve it, we set up an API Key under a special Workfront User, then at the Workfront Portfoio level, created a calculated custom parameter that combined the Portfolio ID with that API Key into a single URL that then opened Hotsheet for "THAT Portfolio, as THAT User", and -- presto! -- after then pasting those links into MS Teams, all Teams Users could see what they needed, pulling in real time from Workfront, without having to resort to using the ("too loose") Public URL.
So, although in your case you're not using one of our Magic Reports (although I'd be happy to chat further on that front via email@example.com if you'r interested), perhaps by describing the technique we invented you can adapt it to use the built in Workfront Reports you're attempting to expose -- whether you then use MS Teams to house it, or just send the URL out "raw" to those who need it.
I'm interested to hear how it goes!