@Koala, just add the proper config in application.json:
http://developer.yahoo.com/cocktails/mojito/docs/intro/mojito_configuring.html#yui-object
Basically, you can do this:
[{
"settings": [ "master" ],
"yui": {
"config": {
"filter": "debug"
}
}
}]
In fact, you can use any yui configuration (http://yuilibrary.com/yui/docs/api/classes/config.html) using the same technique.