Just out of curiosity, what software are you using to burn your DVD-ROM discs in UDF? Also, what version of UDF (i.e. - 1.02, 1.50, 2.00, 2.01) are you using when burning those discs?
If you use Nero Burning ROM (I use version 188.8.131.52), there's an option to burn a DVD-ROM in UDF/ISO mode. I don't see why you need to burn a DVD-ROM using only UDF. Maybe that will solve you and your friend's problem when using B7T9 firmware on your BenQ DW1620 drive. It's worth a try. Let us know if that works.