In the example you give, BATFE had explained, in writing why the Sig arm brace configuration was ok. People said, "But, the law says this. Can you clarify why you're saying it's ok?" And then they reversed position to come into alignment with the law.
In this case, there are already positions on 5320.20 and interstate travel. You aren't asking BATFE "why" they allow what they do, or if, in fact it is in alignment with the law. You're just asking how something works, and what the existing policy is (because they change all the time).
It's not at all the same type of "clarification". People call the NFA branch all day long M-F, 8-5. I've seriously called like at least 50 times.
For example: once I made a DD on a form 1 from a flare gun. I was supposed to put the "original manufacturer of the firearm" on the form. Well, is that me, since a flare gun is not regulated as a title I or II gun and I would be the first person making it a title II gun, or do they still want the original manufacturer of the device in that place?
You can no longer put "multi" as a caliber on an SBR, but you can "temporarily" swap uppers and calibers. If you install a device in the magwell to take a different caliber's magazine, does that constitute a "permanent" change, and would I need to send ATF a letter informing them of the "permanent" change of caliber, or is that still just "temporary"?
They get questions on stuff all day long, and they are not earth shattering and have relatively little effect, and are not similar to the Sig pistol brace case.
When you eat leftovers out of your Mom's fridge during a visit home...that's when it's "better to ask forgiveness than permission". When you're facing the long dick of the law is not the time.
My understanding is that it is not Kosher. I have called the NFA branch and asked this specific question myself, about 7-8 years ago. They could easily have changed their position. My point isn't that I'm right and someone else is wrong, my point is that there is a place where you can get the information directly, and that should not be a buddy or the Internet. Call NFA directly.