<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by PremEx:
But if any F or C passenger wants to be assured an Obento, they may pre-order the same Obento Box as a "Special Meal" at least 24 hours in advance by calling United International Reservations.
Economy passengers can not order an Obento as a Special Meal. 
I think mooching one from C or F as is suggested above might be your only option.</font>
Just a thought...
What if the your travel agent uses Galileo or Apollo as their reservations system? If you happen to know the "code" for the Obento meal and ask them to enter it as a "special meal" even if you are travelling in Economy, what will happen? (each special meal as a code)