Obviously a bit more PR would have helped things along. I don't know if something like this was appropriate for "IT is coming." Probably should have gone with, "SPS is in talks with the Bulldog Club for a possible membership drive, so stay tuned." Of course, they may have asked you to keep it quiet. Also, coming off the huge thread about joining the Bulldog Club, I think a lot of posters probably jumped in before the promotion went into effect. It definitely reminded me to renew for the year, which I guess is why they're in partnership with you now. People are actually joining/renewing/finding out about the Bulldog Club through this site.
Having said all that, I think a standing promo code would be a good idea. So, whenever you want to join or have to renew, you can enter the code and be eligible for a yearly drawing or whatever.
Edit: I like 8dogs idea. Something like an extra point, or even a small bump in the points per dollar donated each year.