I can think of a couple of different approaches you could use to accomplish this. One would be to create a Property in HS Called “Event Attended” or something like that. Then use a one-time workflow in HubSpot to set the property to Yes or No (and perhaps the name of the webinar so you can reuse the field next time), based upon the prospect’s membership in the static list.
Then, create a mirror of that field in Salesforce and make sure that your Salesforce Sync is set up to sync those fields.