I've read about that whole "armor piercing ammunition" fiasco before on AKforum. While I understand doing everything possible to avoid a repeat history, I think it was just a confluence of unfortunate circumstances - with most of the blame falling on the rogue ATF agent who, without legal authority, made the pronouncement. It took some time and threats of legal action, but it was ultimately reversed. Thinking back to 1993/1994, there was a flurry of anti-firearms activity in the executive branch and the legislative branch, and this was just another in a long series of bad laws/enforcement actions.
Practically speaking, I don't see how one person's home build will upset the status quo when it comes to the classification of x54R and 5.45x39 ammunition as "armor piercing" and banned from importation. With some effort, a handgun can be made to fire any ammunition, so the ATF's interpretation and definition is unworkable. I'm not sure that the ATF will want to revisit this issue again when they've already implicitly acknowledged that the earlier interpretation was wrong.
Also, the 1994 ATF pronouncement specifically mentioned production and introducing into the marketplace as being the reason for the re-classification. The rogue agent attempted to clarify this by arguing that the it was because the AK pistol was "offered for sale" (rather than sold in the marketplace), but even under this interpretation, I don't think a home build would constitute that, by any stretch of the imagination.
In any case - this pistol PSL is pretty cool! I've always wanted to build an Obrez, but this may be more of a possibility - I don't know the first thing about welding, much less re-welding a Mosin receiver.