While I know you already called it out, your last point was definitely going to be my answer as well - but I think we can speed up this process with some focused gamification techniques. Depending on the types of missions you create, we can cater them to actions more likely to be taken by a customer than an employee and therefore customers should in theory rank higher, faster even with employees initially having more action items. Obviously in the long run it all takes care of itself, but I understand your focus right at this moment.
Some of things I've seen others do is instead of putting up a leader board right away, they've featured a specific client or two on that homepage or group page. Calling out someone and rotating that featured customer weekly or monthly can help drive more users to champions over employees right out of the gates.