It's not that rare, though I do not know many fly anglers taking a stab at it. But from what I have read, and after some conversations with those who have/do do this, I think a person could swing for them in the salt. Problem might be finding a swingable area without a high density of other fishermen (not flyfishing). The swinger would probably get in the way and piss everyone else off.
Search this forum or google for "whidbey steelhead" and enjoy
I had a nice summer run steelhead buck track my fly right to my knees at Meadowdale beach one summer day. It came so close to eating the fly. Most saltwater steelhead are caught on the west side of Whidbey from mid-November through February. They swim in small schools and enter the shallows very close to the beach. Not easy to catch these days due to depressed steelhead runs, but it can be done.
I use to fish out at Fort Casey in the salt, caught a lot of salmon and more than a few steelhead at that spot. Was virtually unknown until Amato published an article on it in his magazine. Complete with a map of which rock to stand on to cast from. Sorry I did a exaggerate a bit about the rock. I was told about it back in the late sixties with the rule that I was to keep it to myself, but someone broke the rule.
The gear setup is a crazy combo of pink hootchie and spin and glo. Slow swing and retrieve. I had one on a couple years ago with that gear, west side of whidbey. On the same trip, I had one swim right past my feet in about 18 inches of water. I spent a bunch of time trying to swing big gaudy pink steelhead flies, but nothing happened. I think it's doable though. Others probably have much more experience with this.
Had the pleasure of listening to Steve Raymond this past week at our club's meeting. He spoke to the exact question and related that while difficult, it's not impossible. It's much harder now to the dwindling stocks but is still available in estuaries from time to time. His book. -TheYear of the Angler, I think, might have some information. It's very cheap at Amazon.
Steve Raymond discusses fishing for saltwater steelhead in his book The Estuary Flyfisher. He pretty well limits it to estuaries. Small numbers of steelhead and the fact that they don't seem to be as shoreline-oriented as some salmon species means that they are not as frequently encountered along most beach venues. As mentioned above, several well-known beaches along the west shore of Whidbey Island are exceptions to this rule.