There's a couple of variants of this out there, i.e. packcover/poncho combos. I'm a regular user of
a poncho as a packcover, but I use an Integral Designs Silponcho - it's just a big silnylon poncho, what makes it work well as a packcover, and hammock underquilt a-la garrington insulator "taco" is
it's shockcord along the back bottom, so you can snug it under the pack with stretchy shockcord
around your front, and it can blow to beat the dickens out and you don't end up with poncho looking like a helicopter blade and flapping your off the mountain <img src="/forums/images/graemlins/wink.gif" alt="" />

Personally, I'm partial to the plain poncho variety because I also use it as a tarp - and frequently.
along with using it as the aforementioned underhammock windblock/insulation holder (I'm a hammocker). I find it works just fine as a packcover without a pocket or anything special (it's just
long in back) and still works as a tarp and stuff..

I'm sure it'd work fine, just not sure it'd work better than just a long poncho with a shockcord on and draws on the botom of the back. I suppose you might need a pouch if you carry like a 90 litre pack loaded to the gills, but even then I doubt it.