diff --git a/lib/customisation.nix b/lib/customisation.nix
index 101c9e62b9e6..5b203552583b 100644
--- a/lib/customisation.nix
+++ b/lib/customisation.nix
@@ -213,7 +213,14 @@ rec {
             outputSpecified = true;
             drvPath = assert condition; drv.${outputName}.drvPath;
             outPath = assert condition; drv.${outputName}.outPath;
-          };
+          } //
+            # TODO: give the derivation control over the outputs.
+            #       `overrideAttrs` may not be the only attribute that needs
+            #       updating when switching outputs.
+            lib.optionalAttrs (passthru?overrideAttrs) {
+              # TODO: also add overrideAttrs when overrideAttrs is not custom, e.g. when not splicing.
+              overrideAttrs = f: (passthru.overrideAttrs f).${outputName};
+            };
         };
 
       outputsList = map outputToAttrListElement outputs;