I dont think that we can make this kind of poncho extendable enough at the back to cover big packs. We would end with completely doubled rear and thats too expensive, heavy and cumbersome.
For anything larger than, say, 50 Liters, the Nylon ones with a zippered extendable compartment are the ticket. Like these:
http://intranet.tatonka.com/infosys/infocgi/artinfoe.dll?2797&0
I came to this concusion after making these pictures:
The poncho is the standard US one and I choose it because most of you are familiar with the design and dimensions. The pack is a medium-sized Bach, maybe filled with 45 Liters as shown.
In the first pic you see the poncho, for comparison, with the press studs closed all the way down.
In the second I offset the press studs. The lowest on the rear connects to the third from the bottom on front. As you can see, it fits nicely over the pack, completely exposing the legs.
Widht is also reduced by adding depht to the bearer with the pack, but its still comfortable to wear. I mention this, because the Ventile poncho would be narrower (155cm compared to 165cm?) and the fabric is not as smooth as the nylon shown. Therefore it wouldnt "slide" as good as Nylon over clothing and pack.
Now imagine a 80+Liter pack being worn. Being much higher and deeper it would "use" up much more of the back lenght of the Poncho. An extension of more than 1m would be necessary. Far too much in every aspect to carry doubled up at the rear. With a pack this big, I would wear a ventile Poncho under the pack, like a jacket, or get one with a Nylon one with an extendable compartment. Our design has its limits here.
Now, if we were to make a short extension of ca 60cm, a pack like the Bach could be worn under the Poncho AND the protection would be still good down to the shins AND we wouldnt add too much weight- and pricewise.
This extendable flap would be folded inside when not in use. Fastening is a bit of a problem. Press studs would be easy and cheap to fit, but then every time you sit down they may have to be rearranged as not to pinch the bottom. Thats my guess.
Therefore, I would go with ties, at least for the one or two in the middle. On the long sides, press studs wouldnt do any harm. But that can be decided with the genral question of using them or not at all.
If there were no extension at all, this would leave only very small packs to carry under the Poncho.
But then the poncho could always be worn as a long jacket. Which I vastly prefer to any GTX who make me feel like a boil-in-the-bag and leave my legs uncovered, too.
Food for thought, I hope!