Royal Brunei Airlines (RBA) operate a service from Hong Kong to Brunei to Denpasar.
The Hong Kong to Brunei sector is on an RBA aircraft and is a codeshare with Dragon Air.
Don't know about the prices though.
RBA has quite an extensive set of Interline Agreements (IA) and I think the IA with a US Carrier is with United.
RBA's website is
www.bruneiair.com
Cheers
Wings