it is definitely possible to do this. The Marketo forms has a simple Custom HTML that can be displayed if the visitor is known: Show Custom HTML Form for Known People - Marketo Docs - Product Documentation
Hope this helps
If you want to do this on Marketo pages, you can do this directly in your form settings. You have an option to hide the form for known visitors.
Hi Kristan Viox ,
Below is the screen shot of what Floyd and Rachit are referring to. If the Visitor is known, you can replace it with a custom HTML to show something else or maybe a direct option to download a content, depending upon your requirement.
Hope this helps!
For the known visitor you can write any custom HTML as shown in the following screenshot: