"Is it really even possible to get 1:1 pixel mapping on these pseudo-720p displays when showing 1280x720 (16x9) content?"
Not really. But it also isn't really possible on a 1080p panel either.
The 360 does a really good, quick job of scaling though, so if at all possible it is best to have the 360 scale to an exact resolution that your display supports, to avoid any double-scaling, or dog slow display-side scaling. Hence, 768p VGA is the preferred option for 768p displays. In example, my first plasma was 1024x768 42". 360 VGA spit out at 1024x768 16:9 was bloody fantastic and allowed for 1:1 mapping. 720p over component or HDMI was visually inferior in a fairly obvious way, and had forced overscan.