No, I can't point you to any ruling on it.
But, it is perfectly legal.
The only one I know about specifically is the Norell trigger pack.
What might be confusing you here is that ATF considers the trigger pack to be the gun. The action/barrel/stock that you buy at Wal-Mart is not an NFA weapon: the trigger pack is.
This is no different than a drop in auto sear or a lightning link for an AR15. This is just a small drop in part that makes your AR15 run full auto. The registered NFA weapon is that little part. The NFA branch doesn't care about the AR15 you put it in, and you can put it in any AR15 you want or as many as you want: the NFA item is the DIAS or LL.
If this surprises you, have you ever heard of an operational briefcase ?
This is a briefcase that allows you to mount a submachine gun inside and remotely fire the gun while it is inside the briefcase, usually with a remote trigger located on or near the carry handle of the briefcase. Anyway, the ATF considers the empty briefcase to be an NFA weapon and you must have a tax stamp to posses one, even if you don't own the submachine gun that fits inside.
Another example: lets say you buy an HK MP5. Any MP5 you buy will be a coversion done on a semiauto gun. You can get either a registered receiver, or a sear gun. If you get a sear gun, you can take the trigger pack from that MP5 and install it on an HK 91 (.308 battle rifle), or an HK 94 (5.56 rifle) and make them all full auto with your one registered trigger pack. The NFA branch doesn't care about the host gun.