Tonight while at work (pizza delivery), I experimented a little with this.
First off, with my automatic, if I coast (engine on neutral) and then shift into gear when coming to a stop, the car will not engine brake and go into DFCO unless I blip the throttle. Also, usually no matter what gear I'm in or speed I'm going, if I let off the throttle and it doesn't immediately go into DFCO, I can blip the throttle and it will then go into DFCO.
Anyway, tonight I decided to blip the throttle during a few coasts, and sure enough, my ScanGaugeII read 9999mpg after most blips. The problem seemed to be figuring out how hard to blip (light tap or a bit harder tap) and how often to blip (every 2 sec. maybe?). Whether or not fuel was cut off during coasting when my ScanGaugeII read 9999mpg, I don't know, but from what you've tested so far, it seems like blipping the throttle while coasting works.
If further testing confirms this, I like bestclimb's idea on automating it, though I don't know much about setting something like that up.
Nice work.
