custom html for known visitor

Are you using progressive profiling to show addtional form fields for known visitors?

I have a content download page which has multiple pieces of content (stored in one field) which I want know users to be able to pick and choose which content they want - so for known visitors this form field would always be visible.

So I suppose this field could always be shown as a progressive profile field?

You could also use segmentation to create a segment of 'known' leads and a segment of 'unknown' leads. You could use 2 separate forms, the first always showing the additional field to the known leads, while the second form does not include that field for unknown leads.