In that case, annotating the actual type will help bindings use the correct type.

Compare the gir before and after, where the correct type was used:
--- Xeme-1.0.gir    2025-02-14 02:17:09.046001520 -0500
+++ build/xeme/Xeme-1.0.gir 2025-02-14 02:17:35.687145198 -0500
@@ -1333,7 +1333,7 @@
           <doc xml:space="preserve"
                filename="xeme/xememessage.h"
                line="47">The new message.</doc>
-          <type name="Stanza" c:type="XemeStanza*"/>
+          <type name="Message" c:type="XemeStanza*"/>
         </return-value>
       </constructor>
       <method name="add_body"