TL;DR: weapons exactly like this are more of a "cool trope" in fiction than something that actually existed, but they're similar to some real-life ninja weapons which presumably inspired them directly or indirectly.
I found a very informative Quora thread about "long metal claws strapped to your hand" weapons, their usefulness and historical precedents. Several answers give detailed explanations of why such a weapon would not be useful or practical, which explains why they never passed into frequent usage like "long metal blade" weapons did.
However, there were a few weapons originating in eastern Asia, mostly Japanese ninjas, which come close to matching your description:
The Tekko-Kagi, originally a farm tool but adapted by ninjas as a weapon, shaped roughly like a set of bear claws which can be attached either over or under the hand:
The Shuko, another ninja weapon in the form of a row of spikes or claws set inside the palm, and the Bagh Nakh, an Indian weapon shaped like tiger claws and also designed to be held inside the palm:
Of these, the Tekko-Kagi is most similar to the "orchid" described in Dhalgren. It has blades which can sit on top of the hand and be used for slashing and fighting.
However, the key thing to notice is that the Tekko-Kagi blades are relatively short. As you can see in the pictures above, they don't seem to be 8-12 inch long blades. In fact, as one of the Quora answerers explained, such long blades would not be practical to use in combat:
Nope, not useful. Can't punch with it, the end of the blade would push into the fleshy part between your knuckles, same with slashing down, because it would pivot around the handle. The thin blades would also easily get caught in a shield and break and twisted away from user and useless against armour.
It seems more likely that Delany's "orchid" is part of the trope common in various fantasy media of having unrealistically long claws attached to one's hands as weapons. This trope has inspired various forum threads, like the Quora one linked above and this Reddit one, asking about real-world examples or inspiration. Stuff like this looks more similar to the description of the "orchid" than the Tekko-Kagi does:
Although the trope was presumably inspired by the real-life historical ninja weapons mentioned above, the origins in literature of (specifically) the long impractical claws are less clear. I haven't dug deeply into that, since it wasn't exactly part of your question, but it's even possible that Dhalgren was one of the first appearances of such weapons in speculative-fiction literature. The examples listed on the TV Tropes page go back as far as the 1980s, but Dhalgren was published in the 1970s.
And just for fun, that "cool trope" inspired Colin Furze, Youtube's most brilliant mad inventor, to actually build a set of such claws himself. Presumably he doesn't use them as weapons though, so this doesn't speak against their impracticality as mentioned above.
