The Campaigns extension would allow using ?campaign=foo on any page (not just the registration page).
A cookie would then probably be used to keep this campaign for the session allowing the Campaigns extension to then log a future account creation.

This allows WMDE banners that want to track registrations to direct users to some landing page before the user registers rather than having to send them straight to the registration page.

WMDE currently have a 'hack' implemented in the WikimediaEvents extension to do this for them but it would make sense to add it directly to the Campaigns extension.