Then you would or should know about how to cross connect. As an advice I would speak with IB's HK institutional desk. First of all they have the tightest spreads I have seen among any of the ECNs (currenex, fxall, Fastmatch, Lmax,...) and offer deep enough liquidity unless you trade many dozens of million on a clip. Secondly, you may be able to fully circumvent any registration in the US. The client would set up an institutional account with IB and nominate you as account manager with full trading privileges. They may even be able to colocate your client with their Greenwich servers without having to go through any US regulations and registrations but you need to double check that. Good luck in your endeavor
IB can do that also with their F&F account structure. Though if you go the institutional route with them they would have what is called a "Financial Adviser" account structure. At some point you need to actually make a move, pick up the phone and inquire yourself. Unless of course you want to shell out thousands of dollars to have a service provider do that for you. But nobody is gonna assist you for a few hundred dollars, it is not worth their while.