sns: "We are likely going in around May 18-20, and the high water of the last two years make me wonder about how realistic the upstream travel aspect might be."
I can't comment on the portage you're hoping for, but based upon our paddle down the Wawiag this year with higher water, I wouldn't be too worried about an upstream paddle. While we were going downstream, the river is broad and deep and docile enough that you shouldn't have much trouble going upstream.
I'd much rather do the Wawiag upstream in high water than the Nina Moose River in high water, which we did last year and was physically challenging.