Okay, so you understand the penalties involved. The other point I made is I don't know of anyone that has done it or seen a closed welded stock for sale. Maybe the Tech Branch wouldn't have a problem with it. I just don't know. Certainly, there are other pistols for sale with permanently closed stocks for sale. Where the penalties are so severe, I wanted to be clear with that. That and I have shot a roller locked gun in that configuration and it was more difficult (because of the extra weight of the stock than a pistol end cap) to use.
Something else to keep in mind. Most long term members post with not just the members that have posted on the thread in mind. There are also the lurkers that are not going to post but are reading the thread. Another member, possibly years from now might wish to do the same thing the OP wants to do "for the look". That member might not be up on penalties or thought about the extra weight of the welded closed stock would make on shooting the pistol.
Again, I personally think Filing a Form 1 and putting a functional stock (that could be used on more than one firearm) on, would make the pistol so much better and easier to shoot than putting a heavier nonfunctional stock on the back of a pistol for strictly esthetic reasons. Being fortunate enough to own a sear, I have over 20 pistols/barreled receivers that can be used as hosts for the sear. I only have about ten stocks. I keep some of them in a case to bring to the range. I do switch them out as not all the stocks will work with all the pistols. I store the rest of them together away from the pistols.
Someone might very well have written to the Tech Branch as to the use of a permanently closed A3 stock on a roller locked gun. I'm not aware of it. So write a letter. Do not call. I was once told over the phone from someone that worked in the NFA offices in Washington D.C. (this was before they moved) that I could file a Form 1 and register and make a new machinegun. This was 12 or 14 years ago. I asked to have her send that to me in a letter. I never got the letter.
One additional issue that also might be considered is Law Enforcement. The average cop is not well versed in NFA law. I could see possible Law Enforcement "entanglements" if an officer would see a barrel less than 16" with a stock on the end of the receiver. Even if everything is dropped later, arrest, finger prints holding cells, and bail money, don't sound fun to me. And LEO are not well know for gentle care of firearms they feel are a crime. Good luck with the project, whatever you decide to do.
Scott