Yeah, it's been done, it's a good way to fish very slow water that doesn't have enough current to be fished with the traditional downstream swing. Just one thing, tie your fly with lead dumbbell eyes and lots of marabou, but don't ever call it jig, and, whatever you do, remember that your float is not a "float" but a "strike indicator". If you're using one of the commercially made interchangeable-tip lines, just leave off the sinking tip and loop your leader directly to the floating belly section. It may not cast very well (and it may not really be fly fishing), but it does allow you to fish some water that you wouldn't otherwise be able to.