So I've pretty much decided that the most economical and reliable method is just to use a pitot type gauge (similar to bimr suggestion) and a stop watch. Also wanted to clarify some of the other suggestions, the standard hydrant meters that some suggested, are for "selling water" out of a hydrant, and unless Im misunderstood, wouldn't be appropriate for flushing, where you want a high flow (could be 1500gpm). I suppose one could fabricate the device using an appropriately sized turbine meter, but I'd hate to be the one to carry it around, and it would be an awkward fit at best supporting it's own weight from a hydrant connection.
In summary, I think I was wishing for something that doesn't exist, yet.