I found out, that you can update the preview text of a marketing email with the PATCH /marketing/v3/emails/{emailId} endpoint.
PATCH https://api.hubapi.com/marketing/v3/emails/{emailId}
The payload looks like this:
{...'subject': 'My super subject name','content': { 'widgets': { 'preview_text': { 'body': { 'value': 'My preview text' } } }},...}
It works perfectly but it is not documented AFAIK. Can I build my software on top of it or may this be removed at some point?