When I googled for a solution I found a post where it is explained how drag and drop functionality can be added to a YUI calendar widget. I adapted the solution here. Unfortunately I found no better way as to manipulate GRAILS-UI's source code. I edited DataTableTagLib.groovy around line 215 like this:
case 'date': editorConstruction += """ var ${editorName} = new YAHOO.widget.DateCellEditor();\n ${editorName}.subscribe('showEvent', function() {var dd = new YAHOO.util.DD( ${editorName}.getContainerEl());});\n""" break;
With this, the date picker has become draggable and can be of use for my users again.