It's not an ideal solution, but it's something...

Pick a thread that you're interested in but don't have time to read and respond to yet. Post a reply - even a NT reply. It'll show up in [link|http://z.iwethey.org/forums/render/user?userid=56|your] list of posts. When you've got time, click on [link|http://z.iwethey.org/forums/render/user?userid=56|your] login, select the post, edit it or just go back to the thread and read and/or post.

By doing that, you can mark everything read and still go back to threads that you had some interest in. I think Ross and Box have used this technique.

Of course, you do have to pick the threads in advance, but you'd have to do something similar with your suggested technique as well.

HTH.

Cheers,
Scott.