Ed,
You could do that in a number of ways.
If all your entries are on one page, you could add a clientside script to the button that checks all the boxes. Using dojo that could be something like (not tested btw):
dojo.query("input[type=checkbox]", "#{id:yourViewControlId}").forEach( function(checkbox) { checkbox.checked = true;} );
Another approach would be to call a serverside script that fills the selectedIds viewScope variables with all the ID's of the items in the repeat (in the same order !).
Mark