Never done, can't say, but that ratio sounds really small to me, and kind of defeats the purpose of the late extract. Boiling that much extract in that small amount of water runs the risk of burning/carmelization/darkening, I would think.
When I do partial mashes, I use the wort from the grains for most of the boil, and add all the DME in the final 15 minutes. I've never worried about the loss in boil time - the only thing it would affect would be hops utilization and I just don't imagine that the affect is very great.
In any case, doing it the way you suggest would be an interesting experiment and you could report back! Cheers,