I highly doubt the "ground up Kokanee" explanation. There are tailwaters all over the Western United States with reservoirs above that don't experience Kokanee being shredded 24/7 and ejected downstream. Several of those tailwaters that have Kokanee harbor exceptionally large trout near the dams, but that is because of introduced Mysis Shrimp populations that get ejected from bottom release dams (feed for the Kokanee).

Dworshak hatchery records show that the facility has received triploid eggs numerous times.

Were the pre triploid rainbows that had exceptional length/girth ratios lake fish or river fish? It seems even stranger to see a rainbow with those dimensions in a river without some sort of triploid influence.

It's a fine catch on light tackle, and I'm sure a river full triploids would make a decent playground at times.