From the description, it doesn't sound like the idiom has to include the word "loon". I think you could use any idiom you want as long as you put a loon in your quilt.
For example, the idiom "all that glitters isn't gold" could be expressed with a loon that was thread painted with metallic thread -- or that has rhinestones or glitter on it.
Or the idiom to "dance with death" could have a loon dancing with the Grim Reaper. "Have a cow" could be the loon sitting next to its egg that has a miniature cow hatching out of it.
There are tons of sites that list idioms on the web. Here's one if you're interested.
http://www.idiomsphrases.com/