So why not write a bridge between the LW API (serverside) and NNTP (clientside), so you can use it with your favorite NNTP software?

Obviously it would have to be a complex, stateful bridge, probably with own copy of the content and so on. But it's not a priori clear to me that this would require much more work than your original proposal. And it has the great advantage of being unilateral: you don't need to convince anyone else (like the lesswrong.com admins) to do anything.