I think this could be a good idea. Like you said, the aftermarket is not that big for s197s yet, who knows, this could be huge. I do have problems shifting at the right time. I can watch the tach, and head off the road, or I can watch the road, and hit the rev limiter (BITES!). This would solve that problem, so I like what your thinking. This is the first I have ever heard of a hood mounted tach, so at first it does sound crazy. But seeing that they have existed before, certainly helps. If you can design something that does not look ridiculous, I think it may just catch fire.
Something else to consider would be a heads-up display. I was just wondering tonight how difficult it would be to gather info from the computer and display it digitally on the windshield. The absolute simplest solution would be a sift light LED, maybe even mounted on the hood or on top of the dash, more in the line of sight.