I've had a productive coding session in a lucid dream at least once. But it was just a small number of lines of code (a few blocks) I was playing around with, and I had been thinking about it all day. The visual and semantic memory was sort of stuck in my mind, consciously and subconsciously. In any event, it was insanely precise (I was editing on a giant terminal screen) and when I woke up I typed in my refactored algorithm verbatim.

But reading text coherently in a dream is, AFAIU, quite uncommon as you suggest. I used to lucid dream regularly and deliberately as an adolescent. I would hesitate to say you can't focus in a lucid dream--consciously focusing on elements of the dream, or trying to shape the dream, is kind of the point of lucid dreaming. (Of course, what you have most control over is yourself.) But, yeah, you can't usually control things in such fine detail for the duration necessary to coherently manipulate it. Reading any non-trivial text in a dream isn't very common, lucid or not-lucid, AFAIU. Though, I would imagine it's more common for people with experience lucid dreaming.

I'm sure some people can read and write as much as they want, but at a certain point on the curve we're probably dealing with people with abnormal neurology. Sort of like how there's memory palaces and other mnemonic techniques, which anybody can learn, and then there's people with Hyperthymesia.