I really don't see that being an obstacle. I think we would be able to communicate this idea to other SGs no problem. We don't need to be friends to do this, just buy a trophy and have someone keep standings.
They may not like the idea for other reasons, but because they hate us? I don't know. I think if there is a way to ramp up the rivalry and make it more meaningful and exciting for everyone, this is a way. They did it in the Northwest despite the rivalry. I don't think rivals in the US traditionally ignore the other side, they like conflict and argument and competition. In fact if there is a way to rub it in more that they are superior, they will jump to it
I don't know how the other SG's are, but if ESC and VA didn't want to be a part the 'Colonial Cup', then too bad for them. It will reflect poorly on them and fans outside the area will look at them as poor sportsman. However, I doubt it would be an issue.