i don't know if 'old browsers' is the target market, unless there's some funny niche somewhere that can't upgrade their browser.
maybe a neighboring idea would be creating a proxy for low-bandwidth devices -- e.g. transparently compressing/downsampling images, CSS, JS, HTML so that browsing on EDGE is a little more bearable with an iphone or other mobile device. (this might already exist, but i haven't heard of anything like it)
although i guess that wouldn't really use your imagemap hack or screenshot tool :)
As you mention, It would also work well for phones with bad web browsers, or devices who's browsers will become out of date, such as the browser for the Dreamcast.
I suspect there are more people on older browsers than you might suspect- There is a staggering number of people still using browsers which don't natively support simple things like PNG transparency, or Properly nested layers.
Essentially, you could think of it as a last-ditch fallback, which ensures that it displays properly, even the UI experience is no longer ideal.
The nice thing is that you wouldn't need to run it on the client-side.. They could just browse to http://fixthispage.com/news.ycombinator.com and it would give them a version as an imagemap.
maybe a neighboring idea would be creating a proxy for low-bandwidth devices -- e.g. transparently compressing/downsampling images, CSS, JS, HTML so that browsing on EDGE is a little more bearable with an iphone or other mobile device. (this might already exist, but i haven't heard of anything like it)
although i guess that wouldn't really use your imagemap hack or screenshot tool :)