This may or may not be relevant. I was flying from KK (Malaysian Borneo) to Brunei. I had a one way ticket. Malaysian air was unhappy and asked about return arrangements. I had tickets from SIN back home in several weeks time but no ticket from Brunei. The agent asked about funds for transportation from Brunei and I spread out about 6 credit cards. She spoke to a supervisor and I was good to go. (I was prepared to buy a refundable ticket on the spot.) I had no problems with Brunei immigration. They never asked. In fact, I have never been asked upon entry anywhere in SE Asia for departure tickets from immigration officials. I always fill in the time I intend to stay in the country for less than the visa time.

My guess is, it will be the airlines that will give you a problem, not immigration.

I have not been to BKK since the new military govt took over. (I was last there in Feb this year.) It might be different now; they may be less relaxed. I've been to BKK about 6 times and never asked for onward tickets.