I think that's a great idea, and as there are no real Subaru dealerships in my area any more it would be good to find one locally that has good customer relations and can offer servicing and parts for our Subaru's,
Perhaps you can promote your new dealership with some sort of incentives for on the forums like good deal on servicing costs or free MOT when have servicing etc?
As for numbers I think normal for these types of events is around 15 > 25 cars, but if your offering food and area to park may be more?
Best start may be making a new post on this thread asking who would be interested and provide location and details on what will be provided,
Hope this helps,