Nothing illegal as long as they have submitted all the proper paperwork. But you can’t just add them to a trust and allow them to posses the item.Another link - https://www.silencercentral.com/blog/guide-to-adding-a-trustee-to-a-gun-trust/
Nothing in this says that it would be illegal for a "responsible person" to possess the NFA item that is in the trust.
Background Checks for Responsible Persons (Final Rule 41F) | Bureau of Alcohol, Tobacco, Firearms and Explosives
On January 4, 2016, the Attorney General signed ATF Final Rule 41F, Machineguns, Destructive Devices and Certain Other Firearms; Background Checks for Responsible Persons of a Trust or Legal Entity With Respect To Making or Transferring a Firearm, amending the regulations regarding the making or...
www.atf.gov