I just saw this on the Hennessy site. Must be kinda new. Will have to try one next winter:

OverCover for Expedition A-Sym and Ultralight Backpacker A-Sym
The OverCover is uncoated 30D nylon with a breathing hole which fits over the mosquito net. It is intended to maintain warmer temperatures inside the shelter during dry, sub-freezing conditions. $24.95
OverCover for Explorer Ultralite and Explorer DeLuxe
The OverCover is uncoated 30D nylon with a breathing hole which fits over the mosquito net. It is intended to maintain warmer temperatures inside the shelter during dry, sub-freezing conditions. $29.95