In the United States, the Uniform Transfers to Minors Act provides for the possibility of bank accounts, brokerage accounts, and other property to be held in a custodial capacity under the Act so that the custodian has the right to control the property but that legal title is deemed to be in the minor for many purposes.
In the United States Brokerage account agreements under Article 8 of the Uniform Commercial Code create a legal relation known as "custodianship", which is distinguished from the traditional concept of a trust.
The treatment of a brokerage account based IRA as a trust for tax purposes is largely a legal fiction.
To exactly what extent the operation of such a theory would be restricted by Article 8 and the Securities Investor Protection Act in future litigation is not obvious, and is perhaps irrelevant as a practical matter because of the level of detail in the legislation mentioned.
[2] In the United Kingdom, principles of common law operate with greater freedom in relation securities custody.