They used to do the same thing with Friend requests, and SE eliminated that by disabling the ability for free trial accounts to send Friend requests. Would the same thing be viable here?

The biggest concern when restricting access to stuff is to prevent it causing trouble to legitimate new players, so what do people who actually did (or are doing) a free trial think? If Free Trial players could still join parties provided a subscribed player invites them but not the other way around, would that give them sufficient access to seeing how the game works? A Free Trial, by its nature, is already expected to have more limitations than a regular account.