Sorry to be contrary, but I'm not so sure about this. Personally I wouldn't get upset in this particular case. But if it was another company, for an event that I'm not interested in (and if I multiply it by the number of companies that I do business with) then I start to see a problem.
People don't really have the option to unsubscribe from the list, since it might prevent them getting upgrades for your software, so sending out additional announcements on the same list might get some people upset.
Incidentally, do you really think that you'd reach more people than announcements on drobe, ANS, c.s.a.announce etc.. would reach anyway? All of these are places where people expect to receive such information.
Just to be clear, I'm not questioning your motives at all, and I applaud your proactive attitude. I also truly hope these roadshows are going to be a big success. I just feel that your traditional attitude of caution is probably the right one to have.