The river is so big you can follow Sealey's reports along on Google Earth. Major rapids are clearly visible.
Mobax did the Tichigami route solo, and he and I did it tandem. An excellent remote wilderness two week trip that includes huge lake crossings, some up-stream work, and about 11 km of portages. Not really your kind of trip, recped, but we did do some interesting rapids on the Tichigami in particular.
Mobax also did the Marten route last year. This route was used by the Cree, and later the voyageurs and the Rupert Briggade, because it avoids the rapids mentioned by Frankl near the Marten-Rupert confluence, the Gorge at the highway, and the rapids below Lac Mesguez. Lots of flat water, and not that many runnable rapids. Perhaps mobax will comment or respond to a PM.
I've been on Mistissini and Mesguez a number of times, but not the river in between. It's through Cree territory - access and special fishing permits required. See mobax's excellent Tichigami Route description for details about the paperwork. (As an aside, they once asked us where we would be spending the couple of days we had built into our schedule to accomodate bad weather.

). Sealey's description of this upper section of the Rupert does not talk a lot about challenging but runnable whitewater - lots of lake-y sections. I'm thinking about doing it as a fishing trip. It won't be affected by the diversion. At least not until you get down to Mesguez.
From Mesguez on down it's crown land.
I've paddled the section from Mesguez to the route Du Nord a few times. There are lots of good camp sites on the lake near the Rupert egress (I've got a marked-up map somewhere). It takes maybe two short days (could be done in one?) with not much portaging if you can run that long set below Mesguez (visible on maps). It's too much for me, so I do the very long carry on river-right. Somewhere about half-way down there is a place where I've been tempted to ferry across an R2/3 section to what looks like a manageble R2/R3 on river-left for the lower part, but the friggin' river is so big and wide and powerful that I wimp out. And it might not be as benign over there as it looks from river-right. It's a long ways away. A couple of kayakers run this rapid regularly, so I presume it can be done in a covered boat. Not a place to screw-up, though. You'd be in some very bad water on a very big river for a very long time.
Below that there are a couple of mandatory lift-overs and short portages, but nothing too gruelling, and at least one set that l've lined. Fish the fast water below drops and other likely spots, rigged-up for monster pike, but you might tie into some big specks to, or walleye, of course. It's a rare thing to be fishing and catch both big pike and specks in the same water.
Frankl, a 5 lb speck is huge

. Bigger than anything I've got (so far!). I've beat your 15 lb pike a few time, though.

A good place to try for specks is in the fast water near the lip of a drop. Figure they are there hiding from the pike, whose larger size risks them being taken over the edge.
There are good camp sites around the confluence with the Moon, including a seasonal Cree camp and, on the opposite shore of the Moon, a high sandy bank with some flat ground. Excellent. I've seen Caribou there. Fishing that section of the Rupert was not as productive as it should have been, explained by the fishing nets you can see hanging at the Cree camp.
You'll see helicopter landing pads and activity at the site of the dam (there is a lift-over there).
The Gorge at the Route Du Nord, right at the bridge where Frankr started, is spectacular. On one trip one of us, an experienced paddler but with limited whitewater experience, plopped down on the side of the road and started throwing up when he saw it! There is absolutely no doubt that it would kill you. The take-out coming down river to the gorge is on river-left, just after the turn in the river, and only a few hundred yards above the gorge. (you can probably see a little access trail on Google Earth - I don't have broadband so it'd take me a couple of hours to check

)
I've done it in high water with no problem. One time though, at a lower level, there were large whirl-pools and cross-currents with tight eddy-lines and jumbled standing waves that had my partner confused and disoriented. A rather unpleasant time, for both of us. A screw-up more than about ten yards from shore and there'd be no time to get out before the gorge.
I've got marked-up maps of this section.