> There are a lot of documents that would need to be layed out differently after the change.

You could take any font with both "normal" and "light" weights, and then apply the leading and pair-kerning from the normal weight to the light weight, to produce a font that reflows text the same as the normal weight, but uses the ink of the light weight. I do wonder whether the results would cause typographers to gag, though.

I actually think it would be a lot easier to eliminate printing on a document by document basis than changing the font and dealing with all potential issues arising from that.