getParameterizedJobMixIn turned out to be unnecessary.

core/src/main/java/hudson/model/AbstractProject.java

@@ -287,7 +287,7 @@ public abstract class AbstractProject<P extends AbstractProject<P,R>,R extends A
         return buildMixIn;
     }
 
-    @Override public synchronized ParameterizedJobMixIn<P,R> getParameterizedJobMixIn() {
+    private synchronized ParameterizedJobMixIn<P,R> getParameterizedJobMixIn() {
         if (parameterizedJobMixIn == null) {
             parameterizedJobMixIn = new ParameterizedJobMixIn<P,R>() {
                 @SuppressWarnings("unchecked") // untypable

core/src/main/java/hudson/model/ParametersDefinitionProperty.java

@@ -175,7 +175,7 @@ public class ParametersDefinitionProperty extends JobProperty<Job<?, ?>>
         if (delay==null)    delay=new TimeDuration(getJob().getQuietPeriod());
 
         Jenkins.getInstance().getQueue().schedule(
-                getJob(), delay.getTime(), new ParametersAction(values), getJob().getParameterizedJobMixIn().getBuildCause(req));
+                getJob(), delay.getTime(), new ParametersAction(values), ParameterizedJobMixIn.getBuildCause(getJob(), req));
 
         if (requestWantsJson(req)) {
             rsp.setContentType("application/json");

core/src/main/java/jenkins/model/ParameterizedJobMixIn.java

@@ -171,7 +171,7 @@ public abstract class ParameterizedJobMixIn<JobT extends Job<JobT, RunT> & Param
             throw HttpResponses.error(SC_INTERNAL_SERVER_ERROR, new IOException(asJob().getFullName() + " is not buildable"));
         }
 
-        Queue.Item item = Jenkins.getInstance().getQueue().schedule2(asJob(), delay.getTime(), getBuildCause(req)).getItem();
+        Queue.Item item = Jenkins.getInstance().getQueue().schedule2(asJob(), delay.getTime(), getBuildCause(asJob(), req)).getItem();
         if (item != null) {
             rsp.sendRedirect(SC_CREATED, req.getContextPath() + '/' + item.getUrl());
         } else {
@@ -223,10 +223,10 @@ public abstract class ParameterizedJobMixIn<JobT extends Job<JobT, RunT> & Param
      * Computes the build cause, using RemoteCause or UserCause as appropriate.
      */
     @Restricted(NoExternalUse.class)
-    public final CauseAction getBuildCause(StaplerRequest req) {
+    public static final CauseAction getBuildCause(ParameterizedJob job, StaplerRequest req) {
         Cause cause;
         @SuppressWarnings("deprecation")
-        hudson.model.BuildAuthorizationToken authToken = asJob().getAuthToken();
+        hudson.model.BuildAuthorizationToken authToken = job.getAuthToken();
         if (authToken != null && authToken.getToken() != null && req.getParameter("token") != null) {
             // Optional additional cause text when starting via token
             String causeText = req.getParameter("cause");
@@ -251,8 +251,6 @@ public abstract class ParameterizedJobMixIn<JobT extends Job<JobT, RunT> & Param
      */
     public interface ParameterizedJob extends hudson.model.Queue.Task {
 
-        ParameterizedJobMixIn<?,?> getParameterizedJobMixIn();
-
         @SuppressWarnings("deprecation")
         @CheckForNull hudson.model.BuildAuthorizationToken getAuthToken();