What if it is a DIY trust (such as one made from LegalZoom, etc). Would a brokerage firm decline the application? I don't have one of those, but am just curious.
Judging by conversations I have had with brokers, some would decline, because they are uncertain whether the trust will be valid. If you ask them any questions whatsoever they will usually take that as a sign that you do not know what you are doing and will tell you to get a lawyer. This was for a pension trust though, not a living will. Also, they will often ask supplementary questions such as "under what section and paragraph of the CEA are you claiming exemption from CTA registration" etc. and it may be hard to answer these on your own. For a simple living will, though, they might accept it, or possibly you could get a store front lawyer to do one cheaply.
whenever you set up one of these longterm deals, it's important that both you and your kids understand what the relationship is between you and the lawyer. Because by the time it matters, both you and him will either be dead or too senile to figure it out.
We would take any trust agreement that appears to have been validly executed (we are not lawyers and won't look into it). Here in California, we can get in alot of trouble for not accepting otherwise-valid documents.