We've been talking about an off-line builder for years, and if it is possible, I'd really like to see it.
I do all my building off-line myself, directly in the files, but some of the OLC things, in particular Flags, are displayed so cryptically in the files that these parts have to be done on line, which is rather irritating.
I don't think it's an easy task to adapt one of the existing ones to 4D code, however, since our Olc has been changed and added to so much that our zones don't work in any other Circle muds, and zones from those Muds don't work in 4D without heavy conversion, most of which has to be done by hand.
I also don't share Toch's optimism in that having an off-line builder would raise productivity much. With or without extra tools, building is slow and tedious work, and writing the descriptions is what takes the most time. And that's something you already can do in Wordpad, and then run through a spellchecker before copying and pasting them all into OLC.