p:config-upstream-pseudo-trigger is actually applicable to AbstractProject, not only Project.

core/src/main/java/hudson/Functions.java

@@ -47,7 +47,6 @@ import hudson.model.PageDecorator;
 import hudson.model.PaneStatusProperties;
 import hudson.model.ParameterDefinition;
 import hudson.model.ParameterDefinition.ParameterDescriptor;
-import hudson.model.Project;
 import hudson.model.Run;
 import hudson.model.TopLevelItem;
 import hudson.model.User;
@@ -409,7 +408,7 @@ public class Functions {
         return Node.Mode.values();
     }
 
-    public static String getProjectListString(List<Project> projects) {
+    public static String getProjectListString(List<AbstractProject> projects) {
         return Items.toNameList(projects);
     }
 

core/src/main/resources/lib/hudson/project/config-upstream-pseudo-trigger.jelly

@@ -25,7 +25,7 @@ THE SOFTWARE.
 <!--
   Present the pseudo "upstream project trigger". Used inside <p:config-trigger>
 
-  "it" is assumed to be a Project object.
+  "it" is assumed to be an AbstractProject.
 -->
 <?jelly escape-by-default='true'?>
 <j:jelly xmlns:j="jelly:core" xmlns:st="jelly:stapler" xmlns:d="jelly:define" xmlns:l="/lib/layout" xmlns:t="/lib/hudson" xmlns:f="/lib/form">