I might be mistaken Bob but I think those functions are for redirects, rather than grabbing content and including it on the current page.
@letti: there’s likely a way to do it using the api, but it’s way above my head here I’m afraid.
However, have you considered ajax?
If you made your ’current page’ completely blank but for an ajax snippet, then you could pull in the content of the other page randomly using ajax. Something to get you started:
http://modxcms.com/forums/index.php?topic=61483.0
Also - kudos for building this yourself, but why not use Website Optimizer?