If you create a Timeline in Encore and just drop in a Premiere sequence which is, say, 720p, it will create a 4:3 Timeline and letterbox the footage.
Anyway, if you create a Timeline, and drop in something it recognizes as widescreen (say, MPEG-2), and then put in your PP Sequence AFTER that item, it will treat the Sequence properly.
But if you remove the MPEG file, it'll pop back to letterboxed 4:3.
So, if you don't mind putting in some kind of leader -- looks like a single frame of black would do just fine -- then it should work for you.
No, you should not have to do this kind of workaround. But it's something.