People amend trusts all the time. If you can do it for a trust that holds a farm, stocks, houses, why not nfa items. The atf has zero say in the trust laws of any given state. As long as you are in compliance with your state’s respective trust laws, then you are in compliance. I find it of low probability that the ATF would think that they have jurisdiction to disallow any trust properly executed and/or amended.
Heres another example I see all the time that illustrates the issue of an entity owning property; XYZ corporation or LLC owns a set of real property and personal property assets. Often, they just file a certificate of merger and succession with the Secretary of State to change ownership of the entity and all the assets go with it rather than just selling (transferring) the assets directly.
I suspect that you could have a single member LLC which owns one MG34 and simply sell the LLC. There is no form 4 conveyance b/c the LLC still owns the asset. It’s just that Robert now owns the LLC. Is this what the ATF wants; NOPE! Is it probably technically compliant; yes.
The only issue I can possibly see is that if a manager, trustee, officer, etc. has a felony then obviously they should not have access to a firearm.