I linked above to a discussion at length about the issue. The feature does not yet exist, so there's nothing I can add, but as you can see, the next update will enable you to do what you want to do. How to use the feature? We won't know until we can get our hands on it.
Which is why I only quoted the first two, in case @ronald didn't know about them. Standard UI actions which many people use in for Finder file selections but don't realise also work in multi-select enabled list dialogs. Usual reaction is "WTF!?!" when I show show someone how to open three Word docs at once from one "Open" dialog
%PromptWithListModifiers% will be a useful addition.
In the "cogwheel" menu of the "Prompt With List" action.