Wouldn't it be more useful for learning syntax if a more natural English translation were provided? e.g. "We're working on this section today" rather than the stilted "Today we work on this section"?
Would any preposition other than "aan" be possible here? I'd have been tempted to put "in" or "op".
In the ordinary sense, no – 'aan' is the only possible preposition. (If you are physically situated in or on a segmented construction, you could perhaps use 'in' or 'op', but that would only indicate your literal location while you work, not the thing you're actually working on.)