Hi Colin,
I'm afraid there's no default setting override available at the moment. As a workaround, you could adjust the setting in the Datagrid.xml yourself. To do so, just change the widgets/com.mendix.widget.web.Datagrid.mpk file extension to .zip, unzip it, edit the defaultValue in Datagrid.xml, zip it and rename it back to .mpk.
But you'd have to do this every time you update the datagrid from the Marketplace.