3 different stories, that can help:
1- I have already called an outfitter, and asked if, after a run of the Seignelay or Mouchalagane, he would carry us and canoes and gear across the reservoir on any boat. He asked which time of the year, and told me that from in late summer, he would not go since his boat was "only" 25 ft or so, which was not enough for the waves that develop under usual wind.
2- I know that Alain Chevrette has paddled it, and survived. However he warned me about the fact that banks were muddy in places, so either you sleep in the canoe, or you beach on some muddy bank, and have to carry your gear in like 300-500m of mud, to reach the forest cover.
3- Fabien Coulombe and Carrie Wallace have done the Matonipi-Outardes, that you may want to try. You have a tough portage i think from NW of the reservoir, but after that, you are fine.
Their map is available from
www.cartespleinair.orgHope this helps !