One idea for a "Trade overseer" could be something like a unit with these kind of options-
-----------------------------------------------------
Overseer Commands
< Current City > [wood] [clay]...[siege blocks]
<Destination City> [wood] [clay]...[siege blocks]
-----------------------------------------------------
The trader unit would travel back and forth between city 1 and city 2 with the selected resources and the caravans required. If the mentioned resources are not available in a city, he would wait there till they become available and set off again. The trade movements will continue until they are cancelled.
Such a complex mechanism might be an overkill just to keep supplying towns with resources. A lot of players wouldn't be interested in an expensive "Trade Overseer" just for shuttling food and iron.
If trade becomes something more than what it currently is, the arrangements could add some value. Say you need raw materials from two different factions to craft something in your capital and sell it to another faction in need. A few carefully made trade arrangements with the local factions could save a lot of 'work'. This would also justify a high upkeep for a trade overseer unit.
----------------
Also the use of additional caravans for trade outside the player's cities may be restricted. ie the trader would be allowed to carry only 1000 resources or so for any arrangements outside his own cities. This could avoid a few problems like -
1. alliance members setting automated trades to new players, thereby killing their gameplay in 2 weeks.
2. Alt accounts routinely sending resources to the main account and vice versa.
3. A player sending resources to alliance mates while going on a vacation.
Also, a trade arrangement need not involve more than two cities. More cities will make the "orders" page cluttered without any significant addition to functionality.
----------------