You could move it down over the foot to some degree, but not completely. But unless you are REALLY REALLY tall, I don't think it will matter. I am 6'1 and it covers my entire length really. The fact that it is open/uncovered out past your head and foot is not a great source of heat loss assuming you are also using a TQ and the UQ is tight against the hammock.
I have triangle thingies, but they don't really work on the stock NX-250. They end up laying on top of the "tent" and slipping around, or pressing down on the tent too much. If you modified it to use a SRL, then you would put them over it. I find the UQ fits just fine without them though.
Basically I just use the mitten clips on the UQ just outside the gathered ends of my 250, on the whoopie suspension. It makes the UQ elastic somewhat tought, but it works fine.