Display all items in child list when no items are selected in parent list

I’m working on a project management SharePoint site. The site has 3 lists: projects, meetings and tasks. I want to have a web part page that works as follows:

1) Select project from a web part list (let’s call it web part 1)

2) When the project is selected, another web part list (tasks) will display the tasks filtered for that project. If no project is selected in web part 1, nothing is displayed in web part 2.

So far, I have accomplished 1) and 2)

3) Web part 3 is meetings. Given a project, I want this web part to display the meetings filtered by that project. I could do that as well. However what I cannot do is have webpart 2 show all tasks when no meeting is selected. Instead it shows no tasks (and the only way to show tasks is to select a meeting, but there is no way to select all meetings).

Any thoughts on how to get this done? In other words, is it possible to have a master-child relationship and have all items display on the child list when no items are selected on the master list?