What would be more interesting to me is a proofing service where the content is entirely the actual owner's, but the voice and style are tuned by a professional writer. This would smooth out some idiosyncrasies (for better or worse) and make a corporate blog more approachable for customers, without compromising on quality. I imagine most people let their blog posts ferment for a few days, maybe weeks, before releasing them. Outsourcing the polishing and reflecting period would speed things up without cheapening the whole thing.
Having a small, paid audience of writers who also have a technical background to assess posts before they go out would be another way to take it.
Regardless, I suspect you guys will still kill it because there is a need for this in places where they would normally hire an intern to write the blog. Good luck!