cl-api-use.java

public class DepX {
  // stuff which depends upon Lib-version-1.x
}

public class DepY {
  // stuff which depends upon Lib-version-1.y
}

public class UseBothDepXAndDepY {
  DepX depX = ClApi.get(DepX.class);
  DepY depY = ClApi.get(DepY.class);
  // can now call each together even though they use different versions of same Lib
}

cl-api.java

public final class ClApi {

  public static <T> T get(Class<T> clazz) {
    // make a java.lang.reflect.Proxy of clazz where the InvocationHandler impl is below
  }

  private static final class ClInvocationHandler implements InvocationHandler {
    private final ClApi clApi;
    private ClInvocationHandler(ClApi clApi) {
      this.clApi = clApi;
    } 
    public Object invoke(Object proxy, Method method, Object[] args)
	throws Throwable {
      ClassLoader existing = Thread.currentThread().getContextClassLoader();
      try {
       Thread.currentThread().setContextClassLoader(this.clApi.getClassLoader(proxy, method, args));
       method.invoke(proxy, args);
      } finally  {
        Thread.currentThread().setContextClassLoader(existing);
      }
    }
  }  

  private final ImmutableMap<Class<?>, ClassLoader> config;

  private ClApi() { }

  // would be init-ed with some yaml-style mapping of which Class depends upon which Lib-version-xx
  // then for each computed set of ClassLoaders we'd make a mapping of Class to instantiated ClassLoader

  // take in everything from invocation even though we may only need the object's type (may not always be the case).
  private ClassLoader getClassLoader(Object proxy, Method method, Object[] args) {
    return config.get(proxy.getClass()); // TODO - ensure not-Null
  }
}