OK first of all, if landless SL residents are allowed to post events on public venues, then there will be a limited number of public venues to post on, and lots of landless people that might want to post events, so I think the public venues should be limited to only people who do not own land personally.
That being said, my idea is to eliminate the land ownership limitation altogether, and instead implement a permissions system for event posting. First off, if you attempt to post an event on land you don't own, a notification window will be sent to the land owner (or everyone in the group that holds the deed) with all the event information and an option to accept or decline the event. In the case of the land owner group situation, only one person needs to accept.
In addition to that, there should be a separate list from the access list on land that allows everyone on that list, or every group on that list, to host events there. It's important that there's a way to allow everyone in a group to post events. In a club, for example, you might have a group specifically for event hosts, and you'll want all the event hosts in that group to be able to post events there.