Guarantees are tough to come by in Quetico.  While there are a few gems in the park, for the most part the lack of a marked or historic portage to a lake is usually for a reason; i.e. no suitable shoreline for a campsite, poor fishing, water depth, clarity or quality, or just a general low "curb appeal" etc...  I know that doesn't answer your specific questions but FWIW anyway.

I know of some that are great fishing holes and worth a day trip in and out but have found or heard of none that would cause me to make the effort getting into it to spend the night.

Of course, I have an agenda when I go to Quetico and "exploration" is low on the list of priorities.

With the above said, I also don't want to discourage you from doing some bush-wacking yourself.  There was a time when I needed to know what was "over there" but that was before Google earth.  Now I can make an educated guess.

I would be happy to PM my specific, albeit limited experiences to you though.

Lots of historical posts and threads on the topic around here as well.