Override items per page setting

I'm wondering what it would take to override a site's "items per page" setting, particularly when retrieving items via JSON (in this case, a slightly customized JSON feed). Basically, I'd like to show, say 20 items per page in my theme, but get all items when retrieving them via the JSON plugin.

Thanks -- E

I can't think of an obvious way to do this, other than not using an output format at all, but instead creating a new controller/action and getting the items yourself.