Why doesn't this type of stock exist?
For those of us behind enemy lines in AWB states, pre-ban lowers are expensive and it's difficult to find one that's worth a crap. So, what I did was purchase one pristine pre-ban lower for $800 (such a rip off but it was worth it after coming across nothing but sear blocks and Olympic Arms cast lowers for 3 months straight). You can just get various uppers to use with that one pre-ban lower of course, but if you want multiple functioning rifles you either have to drop the coin for another pre-ban lower or configure the rifles in a ban compliant configuration. I think the following stock design would make the latter a more appealing option, as the stock issue is the greatest downside of a ban compliant rifle.
Now, why isn't there a fixed stock model that fits over a carbine receiver extension that comes with spacers to be able to tailor LOP to the shooter? This would not be considered a collapsible stock. It would be legal.
Here's a rough drawing (I'm not an artist).
Basically, the main portion would be just long enough to fit flush with the end of the receiver extension...the same length as a fully collapsed stock. But it would come with extension pieces that securely attach to the back of it, so that you could have whatever LOP you desire. Each extension piece would have a sling cutout as well as a QD connection so that you have attachment points where you want, no matter what length you end up with. Each extender piece could simply screw into the one before it, or connect with some other mechanism. It wouldn't be hard to design. Somebody make this please.
http://i.imgur.com/e6fZr.jpg
ETA: OR, if you wanted to avoid having 3, 4, and 5 separate pieces connected to each other, keep it a two piece design and just offer various extender lengths:
http://i.imgur.com/zspiu.jpg