ORAC,
Harrier II and F-18 have exactly the system you describe. You dial in any fuel quantity you want and when the total fuel gets to that amount you get a 'BINGO' caption (+ bitching Betty in the AV-8B & Hornet). Fuel dumping stops automatically when the bingo caption comes on - the dump switches are held on magnetically. Once you've flown with a system like it, you do wonder why on earth you'd ever design anything else. And that Tornado fuel panel is a total disaster - switches with forwards for 'off' right next to other switches that have backwards for 'off'. Who on earth thought that was a good idea????