There's a mad junction on my way into town where the road joins the main road at about 30 degrees. Van drivers not possessed of windows, and/or necks made of rubber, must swing wildly to the right and then left, to force the proceedings into more of a T-junction. Having failed to do this once while driving a Transit, I attempted to view approaching vehicles using the two square inches of blind spot mirror. It was Extremely Difficult.
When I had my Speedmachine I used to do similar kinds of manoeuvres, because looking backwards over my shoulder was difficult1. I did however have two rear view mirrors, which helped. The rest of the time, such as at T-junctions when I had to emerge between vehicles, I tended to unclip, shuffle back on the seat and pull myself up on the handlebars—which as Kim noted are otherwise very in the way—and push forwards on one pedal as maneouvering demanded.
1 I have the same issue in my Quest, actually. I shuffle back in the seat, sit more upright with my elbows sticking out of the shell, and that lets me twist to view traffic.