How (best) to implement a deployment-time "plug-in" to an EAR in Weblogic?

We have an JEE application that we wish to leverage a "pluggable" component (similar to JDBC drivers) where the implementation classes/jars are external to the EAR. These external classes implement interfaces that  the core application is compiled against (and vice-versa). The initial implementation, though, ran into issues because of the class loader model Weblogic employs. Do you have a recommendation as to the pattern we should be following for this sort of capability? We specifically wish to deploy a "standard" EAR and "add in" the desired library implementation.