Stew-
There are many documented cases of hatchery summer steelhead succesfully spawning in the wild...

There are a couple of rivers in the area that had natural anadramous fish barriers...the WDFW stepped in, dumped some hatchery smolts above these barriers, aided the adults over the barriers on their return and now we have healthy self-sustaining populations.

So if this can happen on a grand scale, I am sure it can happen with a few hatchery fish that decided river gravel was a little more romantic that old men in rubber gloves with plastic buckets!