I have a custom menu on a Google Sheets:
function onOpen() {
var ss = SpreadsheetApp.getActiveSpreadsheet();
var csvMenuEntries = [{name: \"Save as CSV f
I don't think this is supported by the current API.
This isn't supported, but you can add some Unicode symbols to the menu entries' names so they stand out a little from the other options.
Most symbols from this site work: http://copypastecharacter.com/
This is what I've been using to highlight some help menu entries.