I think that I have the solution for you. I was also looking at this issue due the shotgun ban in the also 'wonderful' people's republic of the state of NY. If you have a pistol grip then you cannot use the 10round magazines, only 5 maximum that defeats one of the most attractive aspects of the saiga shotgun in the first place.
The idea is to take a US made stock that has the following characteristics:
1-It is a full block rigid stock.
2-Collapsible even pinned are out, as those by themselves are useless w/o a grip that we know we cannot use in this case anyway.
3-Thumb-hole stocks are not good even if they are USA made.
So IMO I have three options sorted from the most simple and inexpensive to the more complex:
A) Get a thumb-hole US made stock and cut the top slot so it is open. Some of the stocks have the grip in an angle and have enough meat at the bottom that will not compromise the integrity of the stock.
B) A US made polymer stock that is a good candidate to be transformed and work an support piece that goes a little further back and can be molded and bolted to make a secure grip. Then you will need a cheek raiser that will become mandatory to reach the perfect line of sight. I though the contender polymer stock could be a good one but first I need to find out is made here.
C) Have a gunsmith custom cut a new one for you. I would also looks a the hammerhead stock or some other options that suit your taste and ergonomics better.
What needs to be done is nothing new for the also lucky people of the people's republic of the state of california that have been dealing these nonsense bans for a long time.
Here some ideas...
http://www.tenpercentfirearms.com/index.php?main_page=ar15
I have other links of folk's creative ideas to implement an functional and ergonomic yet legal stock for ARs and AK's "EVIL" guns.
I will try to find them and will post them here.
If anyone has an encore and knows if the stock is made in the USA that feedback would be highly appreciated. This might be all I need for the S12.
S12 is a shotgun and doesn't have the same ban restrictions as the AK saiga carbines but in the end the same grip vs something else dilemmas.
Cheers.