Notice I ended up not using the Custom Tags as I mentioned because you wanted just a single item. If you were doing multiple items or wanted variable items per hour you could use custom tags and read that tag instead of the actual Menu Item Name.
This is a great way to do Happy Hour… and BTW @Gerlandog Using this method along with Custom Tags might solve your issue you had long time ago with a trigger initiated Happy Hour… If you were in middle of an order that started before happy hour but it crossed into happy hour before the order was finished… you could simply cancel the order and re-add it and it would have correct price keeping the current ticket and all other items in tact so you would have no need to close the ticket to load a price list.
PS In fact I think this would be the best approach for Happy Hour.