A decent lawyer will be able to draft a trust that will have a change of situs provision. This will allow the trust to move to another state jurisdiction and be subject to that state's laws on trusts. The seems to only be concerned that the NFA devices are legal to have in the state where the trust exists and that the trust is valid under that state's laws. I do simple NFA trusts in Virginia and have had at least one client move to North Carolina without any problems. NFA trusts are not all that complex, but they have to be right and you have to follow the rules laid out in the trust.
The ATF now has examiners that are attorneys whose job it is to read through every trust that comes through the office. They are a bit worried about the prevalence of on-line trusts -- not so much that the trusts might be invalid, they can block those trusts from taking possession until the trust is valid. The big worry of the ATF on trusts set up by non-lawyers is that although the trust may be valid, the people under the trust have no idea what they are doing. More importantly, once the grantors under the trust die the ATF is really worried that people who do not know any better and are "in charge" of the firearms after the grantor and trustees die will just give away NFA weapons or sell them at yard sales, etc. The ATF is not wanting to keep people from having items through trusts, but having a trust that does not provide explicit instructions on what to do when the grantor dies and/or the listed trustees die or are not around is a potential nightmare. From what I gather, the ATF is not looking to put people in jail for having problems with the language of a trust, the seem to be helping people with those sorts of issues. However, I think that the ATF will probably be forced to move with the criminal statutes when the wives or or brothers of people who were trustees under a trust die and they just sell the weapons and put unregistered devices in the stream of commerce.
If you get a trust, make sure you understand it and make sure that whoever might be handling your estate when you die takes a copy of a trust to an attorney upon your death to make sure that federal laws and the rules of the trust are being followed to keep innocent people from having real problems.