Hey Cecile,
Campaign membership cannot be used directly for this sort of the. However, If the approval is stored in a field , you could have it come in as a token on your followup page, and then display particular content conditionally on this. Something like this:
<script>
var approved = {{lead.Approval Status}};
if(approved){
//display content for approved
}else{
//display other content
}
</script>
This would have to be on a Marketo landing page however. If that's not feasible, you would need to use either Marketo SOAP, or determine the display locally in your CMS.