There should be no problem; 1/ make sure there the relation is a reference set. 2/ add a reference set selector to your form with the entity you want to select. 3/ Add a selectform to the "add" button of the referenceset selector. 4/ In the selectform add the "select all" button and set the "allow multiselect" property to true. Now you can select all entries at once with the combination of 'select all" and "select". 5/ Add the "select all" button to the reference set selector and set the "allow multiselect" property to true. Now you can remove all entries at once with the combination of 'select all" and "select".
The problem Patrick is having, is that he cannot set the "Select All" button's 'Selection Type' property to "Select All". Because there is already a Select button.