I assumed that I could change: 

    defaultLayout widget = do
        mmsg <- getMessage
        pc <- widgetToPageContent $ do
            widget
            addCassius $(Settings.cassiusFile "default-layout")
        hamletToRepHtml $(Settings.hamletFile "default-layout")

To:

    defaultLayout widget = do
        mmsg <- getMessage
        pc <- widgetToPageContent $ do
            widget
            addCassius $(Settings.cassiusFile "other-stuff")
            addCassius $(Settings.cassiusFile "default-layout")
        hamletToRepHtml $(Settings.hamletFile "default-layout")

And that it would minify other-stuff.cassius and include it, but this is clearly wrong. I also tried this:

    defaultLayout widget = do
        mmsg <- getMessage
        pc <- widgetToPageContent $ do
            widget
            addCassius $(Settings.cassiusFile "other-stuff")
        hamletToRepHtml $(Settings.hamletFile "default-layout")

Replacing default-layout completely, and it didn't work either. What am I doing wrong here?
-- 
Ian Duncan