Please, please do not not limit this "forum in utero" by making it "registry" -based. My suggestion is "region" or "state" -based. Clubs often host shows and there are shows that are not affiliated with any of the Miniature Horse registries or even any of the Miniature Horse clubs that offer Miniature Horse classes (I can think of several in my current state).
I also think it would be a good idea for an "EVENT Calendar" forum. It would post upcoming events and would be easy to see. And from what I see people like easy and convenient. People would be able to click on each event and give info on or tell about it, and new comers would know what to expect. You might get a few more people at events this way.
The problem with posting on the sale board is it disappears to fast.