Not sure if anyone knows about this little trick, but I just figured out how to run the game in SETA while having it in background mode at the same time.
Basically, all you have to do is play the game on Steam and assign the same shortcut you use to open the Steam overlay to the SETA function. Usually this will be Shift+Tab. It's probably best to leave the regular 'J' shortcut in place. Usually SETA exits whenever any key or key combination is pressed, but this doesn't count if that very keypress is the one activating SETA.
Now your ship will go in SETA mode as soon as you switch to the Steam overlay and stay there while you chat to your Steam buddies and use the Steam browser thingy. You can even tab out and just use your regular browser if you prefer, or just do whatever.
To enter the Steam overlay without turning on SETA, just activate SETA briefly before opening the Steam overlay.
SETA will of course still exit as soon as you dock at a station, cross a gate or decorate the surface of a nearby asteroid.
Now we can all watch our favourite manly cartoon shows while letting our trading ships work for us. Profitsss!
On a side note: A pretty decent update for both Albion Prelude and Terran Conflict has been released, fixing some bugs, rebalancing ships and missions, optimizing stuff and making all music tracks actually play. I love the music. A larger update with more content is said to arrive within a few weeks, possibly the next parts of the Albion plot and the HUB and Player Headquarters.