If you have any interest in driving, don't overlook local driving clubs. They won't care if your horses are registered. Plus they are usually looking for volunteers at their activities, which is a great way to learn!
See if you can find other mini owners in your area with the same interest. We contacted a local show manager and asked her to add some mini classes, which she did as long as we could guarantee 3 entries. We pretty much made up the rules (i.e. any mini under 38" registered or not, measuring not required unless questionable) and sent her show rules from one of the registries. The judge, who had never judged minis before, seemed to have as much fun as anyone!