The only requirements for the POCO are:
1) The object must have a key.
2) The object must be serializable by WCF.
3) The POCO Service Provider class you write must return an IEnumerable<T>.
Provided you know how to dynamically construct the class definition on the client and server, this should work.
Since a dynamic class is essentially used like a map / property bag / dictionary, you may find it easier to define a static POCO type that contains the key-value pairs. This simplifies the type construction, but still gives you access to all the runtime defined data.
If this is a large part of your application, I'm not sure if DevForce is a good fit for you though.