Burn version 2.1u(18) did allow me to burn DVD Studio Pro 3's file using the 16:9 format. My file is very simple: no menus, no chapters, just a short wide screen movie. In DVDSP I set the movie to play 16:9 and loop. In Burn's perfs I made sure 16:9 and loop were set up. Everything works as expected.

Burn is a shareware that works for me now that upgrading DVDSP for the Intel processor has been dropped by Apple.