We want to use a the PageTreeControl with the CMS editor being able to select multiple items from the tree.
How can this be done?
Maybe you could use ContentArea. You will still have single content selection, but the property will store list of content references.
Grzegorz's answer is good. However if you don't need the view rendering that comes with a content area and just want a list of content references then I would suggest having an IList<ContentReference> property. The content selection experience might be slightly nicer with that.