No info on Missinaibi per se, but I was just in the area also (Chapleau & Wawa areas).  Dusk was good off the Chapleau municipal dock!  (see attachment of my 8-yr old's first ever pike ~24")

Fishing was slow from our lodge until we got to smaller, less fished portage lakes.  No monsters, but my boys caught 20+ pike each one day & they were all smiles!  

Personally, I think big lakes like that are hard to fish your first time unless you have good local info.  Certainly fish at dusk (I find dawn & dusk in summer way to hard with 5 am / 10 pm sunrise/sunsets that far north!) & target small, less pressured lakes.  Maybe dawn & dusk in Sept would be easy & certainly few bugs.
-mk631  / Tom

PS: you should probably see this link too...
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