CoPs can in fact be a powerful construct in organizations. A lot of research was already done by the Institute of Knowledge Management (a member organization led by IBM from 1999-2003), and of course as you pointed out Etienne is one of the "fathers" of CoPs. I actually met him several times. He is currently running special programs called Be-treats (http://wenger-trayner.com/betreat/) - onsite but also in virtual versions.

Personally I have found that CoPs (no matter if they were instantiated top-down or just formed bottom-up) usually live and die with a proper CoP leadership. Without a certain type of ownership (by one or a few key drivers), they often will fall apart. The leadership needs to put some pulse into the CoP and have an eye on it, so ongoing value is obtained.
As for the questions of mandated, I fully agree, you can't make somebody think there is value if there isn't for them. Then it becomes just another traditional organizational struture, but you want the fluency of people moving in and out as they get value from the CoP. I also agree, that those at the border can have a high value, even if they only occasionally contribute or use. Innovation often happens at the border of communities, so you need people that move across (almost like a butterfly).
If it is top down it would be the appointment (and funding) of CoP leadership as well as additional funding for face-2-face events that would help to get it off the ground. And management buy-in for people to get the feeling that CoP participation is something wanted and not something "extra". If they find value from the CoP they will get involved, but it is a fine line between that and wasting time sometimes, so basic structure and roles are important to have (even if the roles are not all officially assigned and often are only one of many roles a member might play in an organization).

As Etienne once said, communities, the informal, that is where work really gets done (http://www.co-i-l.com/coil/knowledge-garden/cop/olearning.shtml )

Kind regards,

Frank