Jon, I read this as thoroughly as time allowed on my lunch hour, but it seems like the majority of any of my own thoughts or questions have already been up and addressed in the above posts. I'll just throw out a enthusiastic YES this is much needed, and to the right person, has enormous value.
Let me preface what I'm about to say by saying Signs 101 is a great site. It does however, leave a lot to be desired in the way of business management. I understand that is not it's primary focus or reason so I hope nobody, particularly Fred, takes offense to that. It's just not a business-management oriented site. Rightfully so, too, the audience for people specifically interested in that is limited compared to the audience for people wanting info on printers or mounting techniques or other sign-specific nuts and bolts information or discussions. I don't really understand why that is, we're all here to take our business from point A to point B, but it is what it is. So I think your audience will be limited to a small handful of people who are truly passionate and eager to dive deeper and deeper into the business end of the industry.
The upside to having a somewhat limited group, and a group that paid money to be there, is the information, discussion, willingness to both open your mind to learn new things and willingness to help others, "should" be exponentially greater than what you'd find on a typical discussion board.
I think it's a great idea and I really hope it makes it to fruition, I'll be one of the first to jump on board.