Have you considered the possibility of creating a custom welcoming text for these campaigns? Not removing anything or changing any design from the default page, just being able to add a banner.
Topic on Extension talk:Campaigns
No, we didn't really want to go down the road of custom landing pages or welcome text with this extension. To be honest, I think this kind of easy customization by non-technical folks encourages a fragmented user experience.
E3 is refining a certain kind of Onboarding new Wikipedians flow in Extension:GettingStarted (currently enwiki-only). I can imagine some wiki wanting something different; with either this or the regular Welcomecreation-msg, per-wiki JS could detect the campaign cookie and offer something different for certain values.
This post was posted by SPage (WMF), but signed as S Page (WMF).