It's call short-checking. Some airlines won't allow it, some will at the passenger's choice, and some will force you to short-check if there is a long layover. Maybe check the EK board here?

The whole concept of "hidden city" usually involves an uncompetitive fortress hub airport and a third city where healthy competition exists. Even if you miss a connection, you haven't necessarily committed a CoC violation in the airline's opinion - it typically has to be done for the purpose of evading the fortress hub's fares. I don't know where BKK fits into the grand scheme of things, but as long as your destination isn't someplace with notoriously cheaper fares than BKK fares, it's unlikely to raise any kind of red flag.

Have you priced a multi-city itin? Why not do that, and price the regular A-B fare, and offer to pay the client/employer the price difference between A-B and A-BKK,BKK-B? Seems like that'd be easier than what you propose, which could theoretically find itself rerouted in the case of IROPS. Probably less expensive for you as well, a win-win-win for all.