Thanks for this code. It helped very much.
However, on our instance it has been the case that the field 'page' has been empty at the dashboard tabs, but instead I had to use the field 'canvas_page' to find the related reportIDs. Then the rest of the script worked fine.
As this might help someone else as well, I will just paste the code here. You need to replace the part before the line

var shareUsers = [];

with it:

var dashboardTabM2M = new GlideRecord('pa_m2m_dashboard_tabs');
dashboardTabM2M.addQuery("dashboard", current.sys_id);
dashboardTabM2M.query();
var canvas_pages = [];
while (dashboardTabM2M.next()) {
    canvas_pages.push(dashboardTabM2M.tab.canvas_page.sys_id + "");
}

var portal_widgets = [];

canvas_pages.forEach(function(canvas_pageID) {
    var grid_canvas_pane = new GlideRecord("sys_grid_canvas_pane");
    grid_canvas_pane.addQuery("grid_canvas", canvas_pageID);
    grid_canvas_pane.query();
    while (portals.next()) {
        portal_widgets.push(grid_canvas_pane.portal_widget + '');
    }
});


var reportIds = [];
portal_widgets.forEach(function(portal_widgetIDs) {
    var portals = new GlideRecord("sys_portal");
    portals.addQuery("sys_id", portal_widgetIDs);
    portals.query();
    while (portals.next()) {
        var pageProp = new GlideRecord("sys_portal_preferences");
        pageProp.addQuery("portal_section", portals.getUniqueValue());
        pageProp.addQuery("name", "sys_id");
        pageProp.query();
        if (pageProp.next()) {
            reportIds.push(pageProp.getValue("value"));
        }
    }
});