I think it's a great idea, and I've heard it mentioned by a few people. Of course the hardest part will be rounding people up and getting them to join in the first place. Maybe teach them some SEO stuff or whatever your main service is, then at the end do the traditional webinar thing and say "you can get OK results doing this on your own, but if you REALLY want to bla bla, you'll need to hire me" type of thing.
I want to think that some business owners will hate the idea of a webinar, but you don't care about those people anyway. How do you plan on getting people to attend? The suggestion I've heard was calling the local Chamber and offering to do a class. I tried it once but was actually told "no thanks" which stung a little. Then again, I live in a city full of idiots.
Good luck, I think it's a good idea.
