Liners are ok depending, but i wouldn't want to carry around a liner that weighs almost a pound. Hollow fibers are a great idea, provided the fiber diameter is not on the larger side. There was a study done which compared different fibers as woven into fabrics, and somewhat surprisingly it was found that the smaller diameter solid fibers and their fabrics were actually a bit warmer than the larger diameter semi hollow fibers and their fabrics (this is for synthetics) because they ultimately trapped more air. Furthermore, the triangular shaped fibers trapped more air and thus were warmer than the more round ones.
Silk for example, is quite fine, animal protein, and somewhat triangular shaped so it's quite warm for it's weight, as far as fabrics go and very strong for a natural fabric. A silk liner is also, much, much lighter than the previously mentioned one by Daniel. So if you're looking to prevent draft heat loss near the zipper areas etc, to me it would make more sense to get a silk liner, or possibly a poly microfiber liner.
And if someone has the money, it also makes more sense to just get a bag with a greater fill weight and/or better quality fill. 8 oz of extra, good quality down, will be MUCH, MUCH warmer than a lb of Sea to Summit Reactor extreme liner.
Or as i advised to someone else on another thread, if you need to go the cheaper route, and still add significant warmth at not too much extra weight you can make a small quilt with thin Climashield Apex, and either put it inside your bag (if low humidity), on on top off your bag (with higher humidity, especially if cold). This will add a lot more warmth than the specialized liner, at around the same weight.