The key words in your post are "impounded" and "triploid".
If the fish are trips, so incapable of breeding, and there's little danger of them escaping into rivers, what's the problem with fishing for them in winter ? I have found winter lake fishing for triploid rainbows in the sunny South to be well worth doing, with well conditioned fish catchable using an imitative or semi-imitative approach, why shouldn't the same be true for triploid browns ? About the only drawback with this winter lake fly fishing is the short fishing day, 9:30am to 2:30pm is just about the full extent of when the fish are interested.
But...the factor likely to mitigate against anything other than token stocking with these fish is cost, see Warren Slaney's post on the "stocking" string elsewhere in this section of the site.
I know where you are coming from with the closed season, but the sorts of places likely to stock these fish are commercial ventures that need every penny of income at any time. Let alone in the looming recession, when the first outgoing folks will cut if they have to is discretionary spend on leisure activities