I was wondering if anyone had fished the Winipeg River system in Manitoba for big northern pike 20+ pounds.

I have fished that river system alot for big northerns/walleye/bass, mainly from the Ontario border to Pointe de Bois. Caught some nice pike over the years, biggest to date was 41". There are a good number of them around, and they do get fished pretty hard at times so they seem to be somewhat wise. I haven't fished through the ice, as it is really unstable (and hard to reach)in the area I frequent. They follow typical patterns, creeks/shallow bays in spring, and deeper as the year goes on, coming shallow again in late fall.
If you are new to the area do consider the use of a guide as the reefs/deadheads are extreme.

There are lots, your best bet is staying at one of the lodges or hiring a guide. There were a bunch this pulled out this year over 40", last year a 49" came out (then back in of course).