For the Sig brace, there is a version for AK pistols...
Which you can buy one here..
http://www.jgsales.com/-p-63024.html
Or you can buy the specific sig complete AR here for example..
http://www.budsgunshop.com/catalog/...SIG+Sauer/SIG+PM40011BSPSB+400+5.56+11.5+PSTL
Again, with that brace there meant to be strapped to the forearm, there is nothing to stop the user from pushing the back of that forearm brace into their shoulder and using it in a similar fashion to a standard shoulder stock that would be found on any rifle.
Using the forearm brace that was is not illegal. The brace was not designed to be used that way so using it in a manner it is not "designed" is to take the risk of whatever goes wrong with the application of the firearm when used incorrectly unto yourself.
As far as tax stamps go, you seem to have a bit of a misunderstanding on how the work DVC. You buy a specific stamp for a specific firearm. If you want to buy 5 SBR's each one must have their own stamp purchased if you buying them as an individual. The exception is to use a trust with the stamp purchase. Which even fewer states allow tax stamps through trusts for firearm purchases. Part of getting the tax stamp is specifically registering the specific firearm that the tax stamp will be for. This is because of many reasons such as being required to get permission to take any stamped firearm across state lines.
I will say that a lot of people as of late are buying AR or AK pistols with the buffer tube out the back so they can buy the sig brace, or just buy the whole thing complete. Completely legal firearm and firearm accessory that one can buy without a tax stamp requirement in pretty much all 50 states.
I agree the premise of using the pistol as intended to strap to the forearm for extra stabilization is certainly a "novelty" and of dubious practicality. But there is no law that states people can't decide to use the forearm brace in a way that it wasn't originally designed or intended for. Which people can use it as a shoulder stock legally if they want and use the pistol as one would use a short barrel rifle instead.