Jason, I wish I had more to offer on this topic, but my experiences with imitating and fishing hellgrammites have been underwhelming. I've devised a number of imitations over the years. I've tried several curved imitations (the larvae tend to curl defensively like a salmonfly nymph when drifting), but I was unhappy with the size of the hook required and the large gaff-like gap that resulted. The most promising imitation was a realistic articulated pattern that allowed the use of a smaller hook. Unlike most of my experiments with hinged flies, however, this one was only a mediocre fly. I managed a few decent trout and even a couple of steelhead on the pattern before abandoning it.
Even though I find dobsonfly and fishfly larvae in the majority of my seine samples, I doubt that they ever drift in significant numbers. And I've never witnessed the adults ovipositing on the water. I also doubt that the Woolly Bugger is taken for a hellgrammite even though that was the idea behind Russ Blessing's original. When trout are in a general feeding mode, specific imitation is seldom called for. The Bugger is just an extremely effective suggestion of life that succeeds well under general feeding conditions.
Beyond this, I can share two little tales that may (or may not) be of interest. As a teen, I once spent the night on a large rock in the midst of a fastwater stretch of the Susquehanna at Duncannon. It was the middle of a solitary two-day float, and sleeping on the rock seemed like a good idea at the time. Well, the rock was uncomfortable, my sleeping bag was soaked, and the constant attention of several large dobsonflies made sleep impossible. In the morning, I set off down the river cold, wet, sore, and cursing the abominable insects.
Years later, I was trying to teach some kids, who were participating in a summer camp, to fish (on the Lehigh River). Kids will be kids, and when the fishing (catching) was slow they'd pursue other forms of entertainment. My vain attempt to keep the kids on task was sharply interrupted by the shrill ear-splitting screams that only young girls seem capable of producing. I ran over expecting to see a watersnake slithering into the river. Instead, I discovered that two girls had been digging in the sand along the bank and their excavations had uncovered two large pupating hellgrammites (C. cornuta--the big uglies). I calmed the girls and explained about the alien monsters, but the thing that fascinates me to this day is the color of the larvae. They were bright yellow. This clearly indicated a recent molt, but it also seemed to suggest that hellgrammites undergo a molt while in their terrestrial stage prior to their final transformation into adults. I've never found any information that would confirm this, but I can think of no other explanation.
I'll also pass along a worthless bit of name confusion from the web. Corydalis cornuta is not only the scientific name for a species of dobsonfly but also for a species of flowering herb in the family Fumariaceae!