Why don't you allow Lords to buy and sell food from their regions, without requiring them to be in their regions. The trades could be restricted by distance, however the restrictions would apply to the distance of the trades to the Lords' regions, not to the Lords themselves. This function could be done from the command page, always present, and you could still maintain the current system of character-location based trading on top of it, so that market places would still be useful for longer distance trading.
Also, remember that food exists to check cities from being self-sufficient. Cities already provide high amounts of gold, legions of troops and potent fortifications; the only thing stopping them from being all-important is the fact that food is required to maintain them. A realm of only cities, such as D'Hara, could not only exist, but even thrive, if the dependence on food were to be abolished. So it is an important balancing feature.
What food does not exist to do is provide traders with something to do. So, don't go about this thinking in terms of 'what can we do to give traders something to do.' Traders are an addition to the food system, they are meant to support it; they are not a fundamental pillar of it, it does not exist to support them.
