reportedly, Interactive Brokers; Goldman Sachs; Charles Schwab; Morgan Stanley
The US know your client rules require money laundering checks which, apparently, most brokerages are not set up to do for overseas addresses. The second problem is that the KYC rules require an analysis of whether the client is 'doing the right thing' and 'understands' their options. These two are expensive to implement and mean that most brokerages or banks, such as RBC in London and Goldman Sachs, Morgan Stanley, turn away anything less than USD 500k in the RIA and 2m in all. It's a conundrum.
BTW does it make sense to have many brokers? A good custodian with clear title to the assets solves the security concern, for anything less than a global systemic risk. If one manages all quoted assets through one broker it is easier to negotiate better terms for volume, and co-ordinate the management of risk: although for international clients, one is often forced to use two.