I don't see why you couldn't do that with a facelets template.

There might be some issues with altering the content type and file extension I guess - ask on the facelets list about this (they are very helpful), but writing out the data should be as easy as:

<ui:composition ...>
 <ui:repeat value="#{list}" var="row">
 #{row.a},#{row.b},#{row.c}
 </ui:repeat>
</ui:composition>

Writing out csv files you also have to consider escaping values within which may contain commas.

It's probably easier just to use a library specifically for csv generation, like http://www.csvreader.com/

It sounds like you might want an EL function or even a component to escape things the way you like. If you think this has general appeal, I strongly encourage you to put something together and make it available. You should be able to make it a one JAR add-on that someone can plop into a seam app and use. (I'd really like to see even more community supported libraries like this in the future)